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ELEMENTS  OF 


VEGETABLE  HISTOLOC 


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ELEMENTS  OF 


VEGETABLE  HISTOLOGY 


For  the  Use  of  Students  of  Pharmacy,  Preparatory 
to  the  Study  of  Pharmacognosy 


WITH  62  ILLUSTRATIONS 


BY 

DANIEL  BASE.  Ph.D.. 

Profeaior  of  Chemntry  and   Vegetable    Histology  in  the  Maryland  College  of  Phanmacy,  Department 
Pharmacy,  University  of  Maryland,  Baltimore 


SECOND  EDITION.  REVISED  AND  ENLARGED 


BALTIMORE.  MD. 
THE  AUTHOR 

1905 


-F 


COPYRIGHT,  1905 

BY 

DANIEL  BASE 


PREFACE  TO  THE  FIRST  EDITION 


This  little  book  is  intended  to  serve  as  a  guide  for  beginners 
in  the  study  of  plant  tissues  with  the  microscope,  with  supple- 
mental instruction  from  the  teacher. 

The  greater  portion  deals  with  the  tissues  and  their  arrange- 
ment in  the  higher  plants,  but  it  was  thought  advisable  to  intro- 
duce a  few  lessons  on  the  simplest  plants,  namely,  some  of  those 
of  the  Thallophyte  series,  not  only  on  account  of  their  great 
importance  in  the  life  economy,  but  also  to  show  the  gradual 
increase  in  complexity  of  structure  in  passing  from  the  ex- 
tremely simple  and  minute  plants  of  the  Thallophyte  series  to 
the  complicated  highly  organized  members  of  the  Phanerogams, 
thus  giving  the  student  a  complete  view  of  the  structure  and 
characteristics  of  the  whole  range  of  the  vegetable  kingdom. 

The  knowledge  of  the  tissues  of  the  higher  plants  gained  in 
this  book,  which  maybe  designated  as  the  Junior  Course,  will  find 
practical  application  in  the  recognition  of  official  vegetable 
drugs,  the  detection  of  adulterations,  the  study  of  ground  drugs 
and  their  adulterations.  This  branch  of  study  is  called 
Pharmacognosy  and  is  daily  increasing  in  importance. 

The  opening  pages  deal  with  a  few  physical  principles  and  a 
description  and  explanation  of  the  action  of  a  compound  micro- 
scope, its  defects,  the  requirements  of  a  good  instrument,  etc. 
The  author  is  well  aware  that  it  is  possible  for  one  to  work 
with  a  microscope  without  understanding  its  action,  just  as  a 
man  may  know  how  to  start  and  stop  an  engine  without  know- 
ing anything  of  its  mechanism  and  theory  of  action.  But  it 
needs  no  argument  to  convince  anyone  that  a  student  who 
knows  the  theory  and  structure  of  a  microscope  is  far  better 

(S) 


424 


i ,) 


equipped  than  one  who  merely  knows  how  to  bring  an  object 
into  focus  and  look  at  it.  For  this  reason,  the  subject-matter 
in  the  beginning  of  the  book  was  introduced. 

The  matter  contained  in  these  pages  is  the  outcome  of  the 
author's  needs  in  his  classes.  No  one  book  was  found  entirely 
satisfactory.  All  contained  valuable  material,  but  lacked  other 
matters  which  were  desirable.  Some  were  too  extensive  for 
use  in  an  elementary  course.  In  preparing  these  pages,  the 
author  consulted  a  number  of  books,  to  which  he  desires  to 
acknowledge  his  indebtedness.  The  books  consulted  were: 
Encyclopcedia  Britannica;  Ganot's  Physics;  Behrens'  Botani- 
sche  Mikroskopie;  Bessey's  Botany;  Goodale's  Physiological 
Botany;  Bastin's  College  Botany  and  Laboratory  Exercises; 
Bower's  Practical  Botany;  Huxley  and  Martin,  Practical  Bi- 
ology; Gray's  Lessons  in  Botany. 


September^  1897. 


PREFACE  TO  THE  SECOND  EDITION 


While  the  number  of  chapters  has  not  been  increased  in  this 
edition,  considerable  new  matter  has  been  added  in  several  of 
the  exercises,  and  in  some  instances  better  directions  for  work 
have  been  given.  The  chapter  on  stains,  section  cutting,  mount- 
ing sections,  etc.,  has  been  placed  at  the  end  as  an  appendix. 
Better  illustrations  and  some  new  ones  have  been  provided, 
and  these  have  been  introduced  into  the  respective  chapters 
instead  of  being  grouped  at  the  end  of  the  book,  as  in  the  first 
edition.  Whenever  a  cut  has  been  taken  from  the  works  of 
other  authors,  I  have  made  acknowledgment  in  the  cut.  It  is 
hoped  that  in  its  improved  form  the  book  will  meet  with  the 
approval  of  those  into  whose  hands  it  may  fall. 

September^  1905. 

(4) 


CONTENTS 


THE    MICROSCOPE 

CHAPTER   I. 

PAGE. 

Preliminary  consideration  of  light  raj^s,  plane  and  spherical  mir- 
rors, refraction,  index  of  refraction,  prism,  dispersion, 
lenses,  formation  of  an  image 9-16 

Spherical   and   chromatic  aberration,   achromatic  and  aplanatic 

lenses 17-19 

Simple  microscope,  condition  of  distinctness  of  the  image,  near- 
sighted, far-sighted,  and  astigmatic  eyes,  measure  of  mag- 
nification       19-20 

Compound  microscope,  description  of  parts,  condenser  (Abb6's), 
illumination,  objectives,  dry  and  immersion  objectives, 
angular  aperture  of  lenses ." 21-25 

Oculars,  Huyghens'  ocular,  positive  and  negative  oculars,  method 
of  changing  oculars  and  objectives,  tube  length,  camera 
lucida,  determination  of  magnification  in  a  microscope, 
stage  micrometer,  source  of  light 25-27 

Requisites  of  a  good  microscope — Working  distance,  focal  depth, 

flatness  of  field,  defining  power,  resolving  power 27-28 

Care  of  microscope  and  directions  for  using  same,  accessory  ap- 
paratus   ' 28-29 


VEGETABLE    HISTOLOGY 

CHAPTER   II. 
Linen,  cotton,  silk,  wool,  hemp,  jute,  Manila  hemp 30-33 

CHAPTER  III. 
Yeast  Plant  (Torula),  yeast-cake,  "top"  and  "bottom"  yeast,  Pas- 
teur's solution 33-35 


CHAPTER   IV. 

PAGE. 

Bacteria,  hay  Infusion,  mother-of-vinegar 36-38 

CHAPTER  V. 
Spirogyra,  Hydrodictyon   (Water-net) 39-41 

CHAPTER   VI. 
Reproduction 41-43 

CHAPTER  VII. 

Moulds,  Penicillium  glaucum,  Eurotium  Aspergillus  glaucus,  Mu- 

cor  stolonifer,  Claviceps  purpurea  (Ergot  of  Rye) 43-47 

CHAPTER    VIII. 
Tissues  of  higher  plants,  typical  cell 48-51 

CHAPTER  IX. 

Tissues  of  variously  modified  cells  as  found  in  higher  plants. 

Geranium  stem 52-55 

CHAPTER  X. 
Parenchyma  tissue,  ordinary  and  pitted  parenchyma 55-56 

CHAPTER  XI. 
Collenchyma,  Burdock 57-58 

CHAPTER  XII.  • 

Sclerotic  cells,  walnut  shell,  cocoanut  shell,  cinnamon,  cascara 

and  viburnum  barks 59-61 

CHAPTER  XIII. 
Epidermal  tissue,  stomata 61-64 

CHAPTER  XIV. 

Epidermal  appendages,  stem  of  Geranium  and  leaves  of  Mullein, 

Stramonium,  Digitalis,  Belladonna  and  Hyoscyamus 64-66 

(6) 


CHAPTER   XV. 

PAGE. 

Starches,  various  kinds 67-71 


CHAPTER   XVI. 
Aleurone  grains 72-73 

CHAPTER   XVII. 
Chloroplasts  or  chlorophyll  corpuscles 73-74 

CHAPTER  XVIII. 

Secretion  sacs,  intercellular  air  spaces  and  secretion  reservoirs. . .     74-78 

CHAPTER  XIX. 
Wood  and  bast  fibres,  Geranium  stem,  Cinchona  bark 79-83 

CHAPTER  XX. 
Tracheary  tissue.  Geranium,  Fern,  Pine 84-86 

CHAPTER  XXI. 
Latex  tissue.  Milkweed,  Dandelion,  Chicory 87-88 

CHAPTER   XXII. 
Vasal  bundles  and  types  of  stems,  radial  bundles  of  roots 89-98 

CHAPTER    XXIII. 
Leaves,  bifacial  and  centric 98-100 

APPENDIX 

A.  Various  reagents  used  in  the  study  of  plant  tissues 101-106 

B.  Section  cutting  and  procedure  in  making  a  permanent  mount.  106-111 

C.  More  important  test  reactions  of  the  parts  of  vegetable  cells.  .111-112 


(7) 


ELEMENTS  OF 

VEGETABLE    HISTOLOGY 


THE   MICROSCOPE 

CHAPTER    I. 

Preliminary  Considerations. 

Before  describing  the  mechanical  and  optical  parts  of  a  com- 
pound microscope,  it  is  essential  to  know  something  about  the 
action  of  the  transparent  bodies,  as  prisms,  lenses,  etc.,  on  light 
rays — how  a  lens  forms  an  image  of  an  object,  and  how  the 
image  is  magnified,  etc.;  in  other  words,  a  little  of  the  ele- 
mentary physics  of  light. 

Light  travels  through  homogeneous,  transparent  media,  as 
air,  water,  glass,  in  straight  lines,  and  a  very  narrow  cylinder 
of  light  is  called  a  7'ay,  or  better,  a  pencil  of  light.  Rays  are 
represented  in  geometric  illustrations  by  straight  lines.  The 
fact  that  light  travels  in  straight  paths  may  easily  be  shown  by 
admitting  a  small  beam  through  a  hole  in  a  shutter  into  a  dark- 
ened room  in  which  dust  particles  are  floating  around.  The 
illuminated  particles  will  be  seen  to  lie  in  a  straight  line.  The 
formation  of  sharp  shadows  by  obstacles  in  the  path  of  light 
is  another  evidence  that  light  travels  in  straight  lines.  When 
the  rays  come  from  a  distant  source,  as  the  sun,  moon,  stars,  a 
distant  flame,  they  are  practically  parallel,  and  a  beam  of  such 
light  is  spoken  of  as  parallel  light  or  beam. 

A  convergent  beam  of  light  is  one  in  which  the  rays  come 
together  in  a  point  or  focus. 

A  divergent  beam  of  light  is  one  in  which  the  rays  emanate 
from  a  point  or  focus. 

Mirrors. — When  light  falls  upon  bodies,  it  is  in  general 
reflected.  If  the  surface  be  rough,  the  rays  will  be  reflected  in 
every  conceivable  direction,  each  point  of  it  becoming,  as  it 
were,  a  new  source  of  light.  It  is  for  this  reason  that  rough 
bodies  are  seen,  and  also  from  any  position.  When  the  surface 
of  a  body  is  smooth,  the  light  rays  falling  upon  it  are  reflected 
in  a  definite  direction  according  to  fixed  laws,  and  such  surfaces 
are  called  mirrors.  "Smoothness"  is  a  relative  term,  but  with 
reference  to  light  rays,  it  means  that  there  are  no  unevennesses 
which  are  at  all  comparable  in  size  with  the  wave-length  of  the 
waves  of  light,  which  is  very  small.    When  the  surface  is  plane, 

(9) 


10 


Vegetable  Histology. 


it  is  called  a  plane  mirror^  and  when  spherical,  it  is  called  a 
spherical  mirror.  These  are  the  commonest  forms,  and  both 
are  used  on  microscopes  for  illuminating  objects  examined. 

Plane  mirror. — The  manner  in  which  this  reflects  light  is 
illustrated  in  Fig.  1.  AB  is  a  vertical  section  of  the  mirror, 
CD  is  a  perpendicular  to  the  surface,  OD  is  an  incident  ray, 
and  the  angle  ODC  is  the  angle  of  incidence,  DP  is  the  reflected 
ray  and  CDP  is  the  angle  of  reflection.  It  has  been  shown 
experimentally,  as  well  as  deduced  theoretically  from  the  wave 
theory  of  light,  that  in  a  plane  mirror  the  reflection  follows  a 
fixed  law,  namely,  that  the  angle  of  reflection  is  always  equal 
to  the  angle  of  incidence ;  that  is,  the  angle  CDP  is  always  equal 
to  the  angle  ODC. 


Fig.  1. 


Fig.  2. 


What  is  true  for  one  ray,  as  illustrated  in  Fig.  1,  is  true  for 
any  number  of  rays.  Hence  if  a  beam  of  parallel  rays  of  light 
be  reflected  from  a  plane  mirror,  the  angles  of  reflection  of  the 
rays  will  all  be  equal,  consequently  the  reflected  beam  will  con- 
sist of  parallel  rays.  Similarly,  by  geometrical  construction 
based  on  the  above  law,  it  may  be  shown  that  divergent  rays, 
after  reflection,  remain  divergent  to  the  same  degree  as  before 
reflection.  This  is  shown  in  Fig.  2.  It  follows,  therefore,  that 
the  only  function  of  a  plane  mirror  is  to  change  the  path  of 
light  that  falls  upon  it.  For  example,  on  the  microscope  it 
causes  a  beam  of  light  that  is  received  from  a  window  to  be 
thrown  up  vertically  through  the  object  and  the  lenses. 

Spherical  mirror. — As  indicated  by  the  name,  the  reflecting 
surface  of  this  mirror  is  part  of  a  sphere,  and  may  be  either 
convex  or  concave,  but  only  the  latter  is  of  interest  in  connec- 
tion with  the  microscope. 

In  Fig.  3,  ABD  is  a  section  through  the 
middle  point  B  of  the  spherical  surface,  C 
is  the  center  of  curvature.  The  line  GOB 
through  the  center,  0,  and  the  middle  point, 
B,  of  the  mirror,  is  called  the  axis.  Apply- 
ing the  law  of  reflection,  it  may  be  shown 
by  geometric  construction  that,  in  the  case 
of  a  spherical  surface,  any  ray  AO,  parallel 
to  the  axis  BCG,  will  be  reflected  to  a  point  F  on  the  axis.  This 
point  is  called  the  principal  focus  and  lies  half-way  between 
C  and  B.     It  will  be  seen  that  a  beam  of  light,  consisting  of 


Fig.  3. 


Refraction. 


11 


rays  parallel  to  the  axis,  is  concentrated  or  converged  to  the 
focus,  and  hence  the  illumination  at  the  focus  is  greatly  inten- 
sified. If  a  beam  of  parallel  rays  fall  upon  the  mirror  at  an 
angle  to  the  axis,  it  will  also  be  converged  to  a  focus,  which, 
however,  does  not  lie  on  the  axis  BCG. 

Fig.  4  shows  the  direction  of  reflection  in 
this  case.  This  is  the  condition  that  obtains 
when  the  concave  spherical  mirror  is  used 
on  a  microscope.  It  has  the  additional  prop- 
erty, as  compared  with  a  plane  mirror,  of 
concentrating  light  at  the  same  time  that  it 
changes  its  direction,  thus  producing  a 
much  stronger  illumination  of  objects  to  be  examined. 

Refraction. — It  was  noted  above  that  a  light  ray  travels  in 
a  straight  line.  This  is  true  only  when  the  medium  remains 
the  same.  Light  passing  from  one  medium  into  a  different 
medium  is  bent  out  of  its  course,  still  moving,  however,  in  a 
straight  path  in  the  second  medium,  but  in  a  different  direction 
from  that  in  the  first.  The  bending  of  light  rays  is  known  as 
Refraction^  and  the  action  of  microscopes  depends  on  this  im- 
portant property. 

Fig.  5  illustrates  the  principle  of  re- 
fraction. CD  is  an  incident  ray  of  light 
passing  from  air  into  glass  at  the  surface 
of  separation  ADB ;  DP  is  the  refracted 
ray,  NDS  is  a  perpendicular  to  the  sur- 
face of  the  glass,  NDC  {--a)  is  the  angle 
of  incidence,  PDS  i=h)  is  the  angle  of 
refraction,  and  HDP  is  the  angle  through 
which  the  ray  has  been  deviated  from  its 
original  path,  CDH.  The  angle  of  refrac- 
tion, 6,  for  a  given  angle  of  incidence,  a,  in  air,  varies  for  dif- 
ferent media,  as  glass,  water,  glycerin,  etc.,  hence  the  amount 
of  deviation  of  the  refracted  ray  from  the  original  path  is  vari- 
able for  different  media.  A  ray  passing  from  air  (which  is 
usually  taken  as  the  standard  of  reference)  to  a  denser  medium 
is  always  bent  toward  the  perpendicular  NDS;  that  is,  the 
angle  of  incidence  is  greater  than  the  angle  of  refraction.  The 
incident  and  refracted  rays,  and  the  perpendicular  to  the  sur- 
face, lie  in  the  same  plane.  In  any  medium,  for  example  glass, 
the  size  of  the  angle  of  refraction  varies  with  the  angle  of  inci- 
dence, which  means  that  the  direction  a  ray  will  take  after 
refraction  depends  on  the  slant  with  which  the  incident  ray 
meets  the  refracting  medium.  There  is,  however,  a  constant 
relation  between  the  angles  of  incidence  and  refraction  which 


12  Vegetable  Histology. 

is  expressed  by  the  Law  of  Refraction.    The  law  is  expressed 
mathematically  thus: 

sine  of  angle  of  incidence 

^  n,  a  constant. 

sine  of  angle  of  refraction 

This  ratio,  n,  is  called  the  index  of  refraction^  and  is  differ- 
ent for  the  various  transparent  substances,  but  fixed  for  each 
one.  It  may  be  accurately  determined  by  physical  experiments, 
and  is  usually  taken  with  reference  to  air  as  standard.  There 
is  only  one  position  in  which  light  is  not  bent  out  of  its  path 
in  passing  from  air  into  any  other  medium,  namely,  when  the 
light  rays  are  perpedicular  to  the  surface  of  separation  of  the 
two  media.  The  index  of  refraction  of  water  and  glass  are 
n  =  1.33  water,  n  =  1.52  glass  (crown).  Substances  for  which 
n  is  greater  than  unity  are  said  to  be  more  refracting  than  air. 
The  refractive  index  has  reference  to  light  of  one  color  only, 
i.  e.,  to  monochromatic  light.  The  yellow  light  obtained  by 
holding  salt  in  a  Bunsen  flame  is  usually  employed. 

When  light  passes  from  a  denser  medium  into  air  the  figure 
and  discussion  is  just  the  reverse  of  the  above.  A  familiar 
illustration  of  refraction  is  shown  by  dipping  a  stick  obliquely 
in  water,  when  it  will  appear  bent.  The  well-known  fact  that 
an  object  under  water  is  not  in  the  place  where  it  seems  to  be  is 
also  explained  by  refraction. 

Refraction  by  a  Prism. — In  optics  a  prism  is  any  trans- 
parent medium  comprised  between  two  plane  faces  inclined  to 
each  other.  The  intersection  of  the  two  plane  faces  is  called 
the  edge  and  their  inclination  is  called  the  refracting  angle. 
Triangular  glass  prisms  are  generally  used.  Every  section  per- 
pendicular to  the  edge  is  called  a  principal  section^  which  is  a 
triangle  in  the  case  of  a  triangular  prism.  Fig.  6  represents 
such  a  section,  A  is  called  the  summit  and  BC  the  base  of  the 
triangle.  Let  01  be  a  ray  of  light  falling 
upon  the  prism  in  the  plane  of  the  section 
ABC.  It  will  be  refracted  twice  accord- 
ing to  the  law  of  refraction,  first  as  it 
enters  the  prism  at  I,  then  as  it  leaves  the 
prism  at  D,  so  that  the  direction  of  the  ray 
after  emerging  from  the  prism  will  be  DS, 
instead  of  the  original  direction  OIP. 
The  amount  of  this  deviation  depends  on 
the  size  of  the  angle  of  the  prism,  its  material  and  the  angle  of 
incidence  of  the  ray.  The  angle  PRS  measures  the  deviation 
of  the  refracted  ray  from  the  original  path  and  is  called  the 
angle  of  deviation.  It  is  seen  that  a  ray  of  light  is  deflected 
towards  the  base  of  the  section  ABC,  hence  its  source  O  ap- 
pears to  be  elevated  or  deflected  towards  the  summit  A. 


Lenses. 


13 


Fig.  7. 


Light  rays  of  different  colors  are  bent  by  different  amounts, 
since  the  refractive  indices  for  the  various  colors  of  light  are 
different.  White  light  is  a  combination  of  numerous  colors, 
and  if  a  beam  of  sunlight  falls  upon  a  prism  it  does  not  come 
through  as  white  light,  but  the  constituent  colors  are  refracted 
by  different  amounts,  giving  rise  to  a  band  of  light  containing 
all  the  colors  of  the  rainbow,  namely,  red,  orange,  yellow,  green, 
blue,  indigo,  violet,  red  being  least  re- 
fracted, violet  most.  Such  a  band  of 
colors  is  known  as  a  spectrum.  An  in- 
strument has  been  constructed  for  con- 
veniently observing  the  spectrum  of  white 
light  which  is  known  as  the  Spectroscope. 
It  has  proved  to  be  of  the  greatest  value 
in  chemical  analysis.  Many  new  ele- 
ments were  discovered  by  its  aid,  for  example,  calcium, 
rubidium,  thallium,  indium,  gallium  and  others. 

The  separation  of  the  various  colors,  due  to  the  unequal 
refrangibility  of  the  differently  colored  rays,  is  known  as 
dispersion.  We  shall  speak  again  of  the  unequal  refrangibility 
of  differently  colored  rays  in  connection  with  lenses  and  one 
of  their  defects,  known  as  chromatic  aberration. 

Lenses. — A  lens  is  a  piece  of  transparent  substance  bounded 
by  curved  surfaces  (one  surface  may  be  plane),  and,  according 
to  the  curvature,  it  may  be  spherical,  cylindrical,  elliptical  or 
parabolic.  Lenses  used  in  optics  are  always  spherical  or  ap- 
proximately so.  They  are  usually  made  either  of  crown  glass, 
which  is  free  from  lead,  or  of  flint  glass,  which  contains  lead, 
and  is  more  refractive  than  crown  glass.  In  virtue  of  the 
spherical  surfaces,  a  lens  has  the  property  of  causing  rays  of 
light  which  traverse  it  either  to  converge  or  diverge.  There 
are  six  types  of  lenses  according  to  the  manner  of  combining 
concave,  convex  and  plane  surfaces.  These  are  represented  in 
section  in  Fig.  8. 


Pig.  8. 

They  are  named,  in  the  order  of  the  numerals,  double  convex^ 
plano-convex,  converging  concavo-convex,  double  concave,  plano- 
concave, diverging  concavo-convex. 

Lenses  3  and  6  are  also  called  meniscus  lenses,  from  the 
resemblance  to  the  crescent-shaped  moon.  The  first  three  are 
thicker  in  the  middle  than  on  the  edges  and  have  the  power  of 


14 


Vegetable  Histology. 


converging  light  rays,  the  last  three  are  thinner  in  the  middle 
than  on  the  edges  and  diverge  light  rays;  the  first  are  called 
converging  lenses,  the  latter  diverging  lenses. 

For  our  purpose  it  will  suffice  to  consider  the  properties  of 
the  two  simplest  lenses  of  the  series,  namely,  the  biconvex  and 
the  biconcave,  as  the  general  behavior  of  the  others  of  the  same 
class  is  the  same. 

Biconvex  Lens. — Fig.  9  represents  a  section  through  the 
center  of  a  biconvex  lens.  The  faces  ADB  and  ANB  are  spher- 
ical and  their  centers  of  curvature  are  C  and  C  respectively, 


Fig.  9. 

which  may  or  may  not  be  equally  distant  from  the  center  O  of 
the  lens.  Usually  they  are  equally  distant.  The  line  joining 
C  and  O'  is  called  the  optical  or  principal  axis. 

If  a  beam  of  parallel  rays  fall  upon  the  lens  parallel  to  the 
principal  axis  the  rays  will  be  converged  to  a  point  called  the 
principal  focus,  which,  for  a  lens  of  crown  glass  having  surfaces 
of  equal  or  nearly  equal  curvatures,  coincides  very  nearly  with 
the  center  of  curvature  C.  Of  course,  there  are  two  focal 
points,  one  on  each  side  of  the  lens,  equally  distant  from  the 
center.  This  distance  CO  is  called  the  focal  length  of  the  lens, 
and  it  varies  with  the  index  of  refraction  of  the  glass  and  the 
radius  of  curvature  of  the  faces.  The  shorter  the  radius  of 
curvature,  i.  e.,  the  thicker  the  lens,  the  shorter  is  the  focal 
length. 

Conversely  to  the  above  case,  if  a  divergent  beam  of  rays, 
emanating  from  a  source  placed  at  the  principal  focus,  fall 
upon  the  lens,  the  rays  will  emerge  parallel  to  the  axis.  Fig.  9 
illustrates  this  condition. 

If  divergent  rays  emanate  from  a  point  on  the  axis  anywhere 
between  the  principal  focus  and  a  distant  point,  they  will  be 


converged  after  passing  through  the  lens  to  a  point  on  the  axis, 
as  shown  in  Fig.  10. 


Lenses. 


15 


There  are  any  number  of  such  related  points,  and  they  are 
called  conjugate  foci.  If  the  source  of  light  S  be  moved  away 
from  the  lens  the  focus  S'  will  approach  the  lens  until  it  reaches 
the  principal  focus  C,  which  happens  when  S  is  at  a  very  great 
distance.     On  the  other  hand,  if  the  rays  diverge  from  a  point 


Fig.  U. 

between  the  principal  focus  0  and  the  lens,  they  will  still 
diverge  after  emerging  from  the  lens,  but  less  so  than  before, 
and  hence  will  have  no  real  focal  point,  but  will  seem  to  come 
from  a  point  S'  behind  the  lens,  as  shown  in  Fig.  11.  This 
apparent  focus  is  known  as  a  virtual  or  imaginary  focus. 

Biconcave  Lens. — This  is  just  the  reverse  of  the  biconvex 
lens,  the  spherical  surfaces  being 
turned  inward,  and  rays  of  light 
diverged  or  scattered.  It  has  two 
radii  of  curvature  and  a  principal 
axis.  Rays  of  light  parallel  to  the 
axis  are  diverged,  but  if  the  rays 
were  prolonged  backward  they 
would  meet  in  a  point  or  focus,  as 
shown  in  Fig.  12.  The  lens  has 
never  a  real  focus,  but  only  an  imaginary  one.  The  focus  C 
corresponding  to  rays  parallel  to  the  axis  is  called  the  principal 
imaginary  focus. 

If  rays  diverging  from  any  point  on  the  axis  fall  upon  the 
lens,  after  emerging  they  will  be  still  more  divergent,  and  will 
seem  to  emanate  from  a  point  between  the  principal  focus  and 


Fig.  12. 


Fig.  13. 


the  lens.  As  in  the  case  of  a  biconvex  lens,  there  are  any  num- 
ber of  such  reciprocally  related  conjugate  foci.  Fig.  13  repre- 
resents  two  such  points,  S  and  S'. 

The  focus  S'  approaches  the  lens  more  and  more  as  the  source 
of  light  S  is  brought  nearer  to  the  lens,  and  vice  versa. 


16 


Vegetable  Histology. 


Formation  of  an  Image.  (1)  By  double  convex  lens. — 
Without  describing  the  geometric  construction,  the  following  is 
the  fact :  That  when  a  small  object  is  placed  near  the  principal 
focus  but  a  little  distance  in  front  of  it,  the  image  formed  is  at 
a  great  distance,  is  inverted  and  much  larger,  and  that  in  pro- 
portion as  the  object  is  nearer  the  principal  focus.  This  is 
shown  in  Fig.  14,  where  the  arrow  A  represents  a  bright  body 
and  the  arrow  B  its  inverted  image,  much  larger  and  at  a  great 


Fig.  14. 

distance.  C  is  the  focus,  the  body  being  a  little  beyond  it.  The 
rays  of  light  coming  from  every  point  of  A  are  converged  by  the 
lens  to  a  corresponding  point  in  the  image  B,  and  the  latter  is 
real  and  can  be  caught  upon  a  screen  held  at  B.  The  figure 
represents  what  takes  place  in  a  compound  microscope,  as  will 
be  shown  later.  The  object  and  image  have  the  same  propor- 
tion as  their  distances  from  the  lens. 

When  the  object  is  very  near  the  focus,  but  between  it  and 
the  lens,  the  rays  from  the  various  points  of  the  object  are  not 
converged  to  a  corresponding  point  as  in  the  previous  case,  but 
pass  through  the  lens  still  diverging,  but  less  so  than  before, 
and  in  such  a  way  that  if  they  were  prolonged  backward  they 
would  meet  and  form  an  image  behind  the  object  on  the  same 


r-i 


J?- 


Fig.  15. 


side  of  the  lens  as  the  latter.  Fig.  15  will  illustrate.  An  eye 
held  in  front  of  the  lens  would  see  the  light  coming  from  the 
object  A  as  if  it  came  from  B,  and  B,  therefore,  is  the  image  of 
A,  erect,  larger,  but  unreal  or  imaginary. 

A  lens  used  in  this  manner  to  form,  with  the  help  of  the  eye, 
an  erect  magnified  image,  constitutes  a  simple  microscope  or 
magnifier.  In  both  cases  considered,  the  magnification  is 
greater  according  as  the  object  is  nearer  the  focus  and  the  focal 
length  is  decreased,  i.  e.,  the  lens  is  more  convex.     Magnification 


Spherical  Aberration.  17 

may  also  be  increased  by  combining  two  or  three  lenses  into  a 
"system." 

There  is  a  third  position  which  the  object  may  occupy, 
namely,  the  principal  focus.  In  this  case  no  image  at  all  is 
formed,  because  rays  diverging  from  any  point  of  the  object 
emerge  from  the  lens  parallel  to  one  another,  and,  therefore, 
have  no  focus  or  image.  The  effects  produced  by  a  lens  for 
the  three  positions  described  may  readily  be  verified  by  slowly 
moving  a  lens,  lying  on  a  printed  page,  vertically  upward,  when 
the  letters  will  first  appear  erect,  then  vanish  and  finally  return 
in  an  inverted  position. 

(2)  By  double  concave  lens. — No  real  image  is  ever  formed 
by  a  concave  lens,  because  it  never  forms  a  real  focus;  it  is 
always  virtual,  erect,  smaller  than  the  object  and  nearer  the 


Fig.  16. 


lens  than  the  object.  If  A  be  an  object  in  Fig.  16,  the  rays  of 
light  coming  from  any  point  of  it  will  emerge  from  the  lens 
more  divergent  than  before,  and  will  appear  to  the  eye  to  come 
from  a  point  nearer  to  the  lens  and  to  the  axis  as  shown. 

Concave  lenses  are  not  used  in  the  microscope  except  in  com- 
bination with  convex  lenses  in  some  "objectives." 

Spherical  and  Chromatic  Aberration. — These  are  two  seri- 
ous inherent  defects  in  all  simple  lenses.  They  are  detrimental 
to  the  formation  of  a  perfect  image  of  an  object  and  must  be 
approximately  overcome  in  compound  microscopes  if  these  are 
to  be  of  any  value  at  all. 

Cause  of  Spherical  Aberration. — It  was  said  above  that 
parallel  rays  of  light  falling  upon  a  double  convex  lens  are  con- 
verged to  a  point,  but  this  is  not  quite  true.  The  rays  falling 
on  the  edge  of  the  lens  are  brought  to  a  focus  nearer  to  the  lens 
than  those  rays  falling  near  the  center  of  the  lens,  so  that  the 
rays,  instead  of  coming  together  in  a  point,  are  focused  over  a 
small  circle.     The  diagram  (Fig.  17)  will  illustrate. 


18  Vegetable  Histology. 

The  rays  around  the  axis  of  the  lens  will  meet  in  a  focus  0, 
while  those  near  the  edge  will  meet  in  C,  and  a  screen  placed 
at  C  will  not  receive  a  mere  point 
of  light,  as  would  be  the  case  if  the 
lens  were  perfect,  but  a  small  cir- 
cle of  light.  The  result  of  this  is 
that  the  image  of  any  object  is  not 
sharply  defined,  but  is  somewhat 
blurred  in  such  a  manner  that 
when  the  center  of  the  image  is  p.     ^^ 

sharp  the  edge  is  indistinct,  and    i  -■ 

when  the  edges  are  sharp  the  center  is  indistinct.  This  defect 
is  due  to  the  spherical  nature  of  the  lens,  hence  its  name.  As 
the  edge  rays  are  most  effective  in  causing  this  aberration,  the 
latter  can  be  greatly  corrected  by  cutting  out  the  edge  rays  by 
means  of  a  diaphragm  or  perforated  disc  placed  in  front  of  the 
lens.  This  is  done  in  objectives  and  eye  pieces  of  compound 
microscopes. 

Mathematical  calculation  has  shown  that  spherical  aberra- 
tion is  greatly  reduced  when  the  radii  of  curvature  of  a  lens  bear 
a  certain  ratio  to  each  other,  namely,  6  :1,  the  face  with  longer 
radius  being  turned  towards  the  object.  Aberration  is  also 
corrected  in  part  by  combining  several  lenses  of  suitable  curva- 
tures into  a  system,  the  lens  next  the  object  being  plano-convex, 
with  the  plane  face  towards  the  object.  (Absolute  correction 
for  spherical  aberration  is  impossible.) 

Cause  of  Chromatic  Aberration. — We  have  seen  that  rays 
of  different  colors  have  different  indices  of  refraction,  i.  e., 
unequal  refrangibilities,  so  that  if  white  light  is  passed  through 
a  prism  the  constituent  colors  are  separated  by  it  into  a  spec- 
trum. A  lens  acts  like  a  prism  in  this  respect ;  in  fact,  it  may 
roughly  be  considered  as  two  prisms  with  their  bases  together. 
There  is  a  different  focus  for  each  of  the  seven  different  colors 
composing  white  light;  violet  being  most  refracted,  is  focused 
nearest  the  lens,  while  red,  being  least  refracted,  is  focused 
farthest  from  the  lens  (Fig.  18).  The  red  rays  will  meet  at 
R,  the  violet  ones  at  V,  and  the  other  colors  at  points  inter- 
mediate, in  the  order,  orange,  yellow,  green,  blue,  indigo.  The 
result  of  this  defect  is  that  the  image  of  an  object  is  bordered 
by  a  color  fringe  instead  of  being  perfectly  colorless,  as  it 
should  be.  Chromatic  aberration  is  more  perceptible  in  pro- 
portion as  the  lenses  are  more  convex,  i.  e.,  as  the  magnifying 
power  increases.  It  is  corrected  by  combining  lenses  made 
from  crown  and  flint  glass.  The  refractive  indices  of  these  are 
very  nearly  the  same,  being  1.751  for  flint  and  1.53  for  crown, 
but  the  power  to  separate  the  colors  of  white  light  is  nearly 
twice  as  great  for  flint  as  for  crown  glass.  Hence  a  biconcave 
or  plano-concave  flint  glass  may  be  so  combined  with  a  biconvex 


Distinctness  of  Image. 


19 


crown  glass  that  the  dispersion  of  one  is  corrected  or  compen- 
sated by  that  of  the  other,  while  the  two  still  act  like  a  double 
convex  lens  in  magnifying  the  object  (Fig.  19).  Chromatic 
aberration  cannot  be  corrected  absolutely;  there  will  always  be 
a  htt  e  color,  but  it  may  be  so  little  that  the  image  is  practically 
colorless.  For  optical  purposes  the  blue  and  orange  are  cor- 
rected or  combined.  If  the  image  is  bordered  by  a  light  blue 
fringe,  the  lens  is  said  to  be  overcorrected ;  if  by  a  reddish  one 
it  IS  undercorrected.  ' 

A  lens  free  from  chromatic  aberration  is  called  Achromatic 
and  one  free  from  both  spherical  and  chromatic  aberrations  is 
called  Aplanatic. 


Fig.  18. 


Fig.  19. 
a— crown  lens. 
1>— flint  lens. 


Simple  Microscope. — This  is  nothing  but  a  convex  lens  used 
as  a  magnifier  as  described  under  double  convex  lens.  There 
may  be  one  lens  or  several  combined  into  a  system  and  mounted 
in  a  suitable  stand.  Corrected  lenses  and  diaphragms  may  be 
used  to  get  rid  of  spherical  and  chromatic  aberrations.  A  good 
example  of  a  simple  microscope  is  a  reading  glass  or  a  watch- 
maker's magnifier. 

Condition  of  Distinctness  of  the  Image. — There  is  for  each 
person  a  distance  of  most  distinct  vision^  a  distance  at  which 
an  object  must  be  placed  before  the  eye  to  be  seen  with  greatest 
distinctness.  This  distance  is,  for  the  average  eye,  between  12 
and  14  inches.  It  differs  for  different  observers,  and  the  two 
extremes  are  found  in  near  and  far-sifted  persons.  When  an 
object  is  looked  at  through  a  lens  the  latter  must  be  moved 
back  and  forth  until  the  image  is  formed  at  the  particular 
observer's  distance  of  distinct  vision,  and  this  operation  is  called 
focusing.  This  explains  why  two  persons  looking  through  a 
microscope  will  have  quite  different  foci,  since  the  two  eyes 
have  different  distances  of  distinct  vision. 

The  human  eye  is,  in  one  respect,  an  optical  instrument,  be- 
cause it  consists  of  a  combination  of  lenses  which  focus  upon 
the  retina  light  rays  coming  from  illuminated  objects.  The 
eye  has  but  little  spherical  aberration,  owing  to  its  peculiar 
shape  and  to  the  action  of  the  iris,  which  takes  the  place  of  a 
diaphragm ;  but  it  does  have  considerable  chromatic  aberration. 
Most  eyes  have  power  of  accommodation,  that  is,  of  altering 


20 


Vegetable  Histology. 


their  focal  length  at  will  so  as  to  perceive  objects  at  different 
distances  away.  There  are,  however,  several  possible  optical 
defects  in  eyes,  which  may  arise  from  various  causes : 

1.  The  rays  may  be  brought  to  a  focus  in  front  of  the  retina 
instead  of  on  it.  Such  eyes  are  called  near-sighted^  and  may  be 
helped  by  the  use  of  diverging  lenses,  which  cause  the  rays  to 
become  less  divergent  in  the  eye  and  thus  to  meet  in  a  focus 
farther  back  on  the  retina. 

2.  The  rays  may  be  focused  back  of  the  retina.  Such  eyes 
are  called  far-sighted,  and  may  be  helped  by  the  use  of  con- 
verging lenses,  Avhich  act  in  a  manner  oposite  to  that  stated  for 
diverging  lenses  in  1. 

3.  The  focus  may  be  different  for  different  sections  of  the 
eye.  If  the  dial  of  a  clock  be  looked  at  an  eye  may  see  the 
figures  2  and  8  clearly,  but  may  not  see  the  5  and  11  sharply. 
Such  eyes  are  called  astigmatic,  and  may  be  helped  by  the  use 
of  cylindrical  lenses.     (Ames'  Theory  of  Physics.) 

Measure  of  Magnification  in  a  Simple  Microscope. — The 
apparent  magnitude  of  an  object  is  the  angle  it  subtends  at  the 
eye  of  the  observer.  In  the  case  of  two  objects  seen  at  the  same 
distance,  the  ratio  of  the  apparent  diameters  is  the  same  as  that 
of  their  absolute  magnitude.  Hence,  in  a  simple  microscope 
(also  in  a  compound  one),  the  magnification  is  equal  to  the 
ratio  of  the  apparent  diameter  of  the  image  to  that  of  the  object, 


Fig.  20. 


both  being  at  the  distance  of  most  distinct  vision.  But  as  the 
apparent  diameters  are  not  easy  to  measure,  a  simpler  method 
is  used  which  gives  an  approximate  measurement  (Fig.  20). 
AB  is  an  object  and  A'B'  its  image,  formed  at  the  distance  of 
distinct  vision  for  the  eye  E.  Since  the  eye  is  always  very  close 
to  the  lens,  the  angles  subtended  by  the  object  and  image  may 

be  taken  as  A'OB'  and  aOb,  and  the  magnification  =  ^ 


This  is  approximately  equal  to 
A'B' 


B'      A'B' 


ab        AB 
DO  (dist.  of  distinct  vision) 


aOb  ■ 
,  and  by  similar  tri- 
12  to  14  inches 


angles,  ^^^  ^^  ^^.^^^  ^^  object  from  lens)  CO  (focal  length)' 
since  the  object  is  very  nearly  at  the  focus.  Hence,  magnifica- 
tion by  convex  lens  =  ratio  of  distance  of  distinct  vision  (say 


Compound  Microscope. 


21 


13  inches  as  average)  to  the  focal  length  of  the  lens.  It  will 
be  seen  that  magnification  is  greater  as  the  focal  length  is 
smaller  and  as  the  observer's  distance  of  distinct  vision  is 
greater. 


COMPOUND  MICROSCOPE 

The  simplest  form  would  consist  of  two  simple  microscopes  or 
magnifiers,  one  with  short  focus,  placed  near  the  object,  called 
the  objective,  the  other  with  longer  focus,  placed  next  the  eye, 
and  called  the  eye-piece  or  power.  The  objective  forms  an  in- 
verted real  image  of  the  object,  and,  by  means  of  the  eye-piece, 
we  see  a  virtual,  erect,  magnified  image  of  the  real  image. 


Fig.  21. 


eye-|biece 


Mode  op  Action. — In  Fig.  21,  AB  is  an  object;  an  image  is 
formed  at  ab,  real,  inverted  and  magnified.  The  eye-piece 
forms  an  imaginary,  erect,  magnified  image  of  ab  at  A'B'.  This 
is  the  principle  of  all  compound  microscopes.  This  form  would 
be  very  defective  on  account  of  spherical  and  chromatic  aberra- 
tions, and  we  will  now  studj^  the  more  perfect  microscope. 

Description  of  Compound  Microscope. — Fig.  22,  A  is  the 
base;  B,  pillar;  C,  pillar  and  arm;  D,  body;  E,  nose-piece;  F, 
objective;  G,  ocular;  H,  draw-tube;  I,  collar;  J,  rack  and  pin- 
ion ;  K,  coarse  adjustment ;  L,  fine  adjustment ;  N,  spring  clips ; 
O,  mirror;  P,  mirror  bar;  Q,  diaphragm  and  substage;  R,  sub- 
stage  screw;  S,  stage;  T,  pillar  hinge-joint. 

Only  a  few  words  need  be  said  about  the  mechanical  parts  of 
the  instrument,  as  the  figure  will  explain  sufficiently. 

By  the  rack  and  pinion  movement  K,  the  body  D  is  given  a 
large  up-and-down  motion  and  a  body  is  quickly  brought  into 
rough  focus.  Then  by  the  micrometer  screw  L  the  fine  adjust- 
ment is  made,  a  very  small  motion  of  the  body  D  being  pro- 
duced by  one  turn  of  the  screw. 

The  draw  tube  H  carries  a  scale  so  that  any  tube  length  can 
be  obtained  by  pulling  out  or  pushing  in.     The  ocular  G  slips 


22 


Vegetable  Histology. 


Fig.  22. 


Objectives. 


23 


into  the  end  of  the  tube  H.  The  triple  nose-piece  E  is  a  con- 
venience for  sliding  one  or  the  other  objective  into  place  as  de- 
sired. The  stage  is  perforated  in  the  center  for  transmitting 
light,  reflected  up  by  the  mirror  O.  In  the  opening  there  may 
be  fitted  little  cylinders  with  smaller  openings,  known  as 
diaphragms,  the  object  of  which  is  to  regulate  the  amount  of 
light.  There  is  a  series  of  three  or  four  of  these.  On  the  stage 
are  two  clips  for  holding  a  glass  slide,  on  which  the  object  is 
examined. 

The  iris  diaphragm  Q  is  much  more  convenient  than  the 
cylinder  diaphragms,  as  the  opening  can  be  made  gradually 
larger  or  smaller  by  simply  turning  a  small  lever  back  or  forth. 

If  a  greater  concentration  of  light  is  desired  than  is  produced 
by  the  concave  mirror  O,  a  condenser  is  used,  which  is  placed  in 
position  beneath  the  stage.  The  best  form  is  the  Ahhe  type 
(Fig.  23),  consisting  of  one  lens  or  a  system  of  lenses  for  con- 
verging a  large  beam  of  light.  The  condenser  is  used  for  great 
magnification  and  is  invaluable  in  studying  stained  specimens, 
which  are  to  be  differentiated  by  color  rather  than  by  outline. 


Condenser  of  1.20  num.  apart. 


Condenser  of  1.40  num.  apert. 


Fig.  23. 


Illumination. — No  fixed  rule  can  be  laid  down  in  regard  to 
the  size  of  opening  in  the  diaphragm  to  be  used  for  any  given 
magnification,  as  the  amount  of  light  to  be  passed  through  a 
specimen  depends  somewhat  on  its  nature  and  thickness.  As  a 
general  rule,  large  diaphragms  are  used  for  low  powers,  with 
weaker  illumination  and  small  ones  for  high  powers  with  strong 
illumination.  Weak  illumination  is  brought  about  by  the 
plane  mirror,  stronger  by  the  concave  mirror  and  the  use  of  a 
condenser  if  desirable.  Actual  laboratory  practice  is  better 
than  many  words  in  teaching  the  student  what  is  the  best 
illumination  of  an  object. 

Objectives. — The  objectives  are  the  most  important  parts  of 
the  whole  microscope.  Instead  of  one  lens  they  consist  of  a 
system  of  two,  three  or  four  lenses,  some  of  which  are  simple, 
others  compounded  of  a  convex  crown  lens  and  a  concave  flint 
lens,  as  described  under  chromatic  aberration.  The  front  lens 
of  the  system  always  has  a  plane  face  which  is  turned  towards 
the  object,  and  is  usually  a  simple  lens  (plano-convex).     Such 


24 


Vegetable  Hisi'Ology. 


a  system  of  lenses  is  almost  free  from  aberration  defects. 
According  to  the  method  adopted  by  the  maker,  objectives  are 
designated  by  letters,  as  A,  B,  C,  etc.,  or  by  numbers,  as  1,  2,  3, 
etc.,  or  by  figures  which  represent  focal  lengths.  In  the  latter 
method,  which  is  the  most  rational,  if  an  objective  is  marked, 
say  1  inch  or  ~/.^  inch,  this  means  that  its  magnifying  power  is 
the  same  as  that  of  a  simple  lens  whose  focal  length  is  1  inch  or 
Vs  inch.  In  order  to  know  which  is  high  or  low  power,  the  stu- 
dent should  remember  this  rule:  TUe  smaller  the  nu7nher  or 
fraction  representing  the  focal  length  of  an  objective^,  the  greater 
is  its  magnifying  power.  The  same  rule  applies  to  oculars  or 
eye-pieces.  The  objectives  mostly  used  in  vegetable  histology 
are  those  of  1  inch,  Vg  inch  and  Ve  ^^^^  focal  length.  The  dis- 
tance between  the  front  lens  of  the  objective  and  the  object 
when  in  focus  is  about  equal  to  the  focal  length  of  the  objective, 
and  is  known  as  the  working  distance. 

Objectives  are  either  dry  lenses  or  immersion  lenses.  If,  as 
is  usually  the  case,  there  is  an  air  space  between  the  objective 
and  the  object,  the  lens  is  called  a  dry  one ;  if  a  liquid  is  between 
the  object  and  the  lens,  the  lens  is  called  an  immersion  lens. 
The  liquid  may  be  water  or  an  oil.  If  water,  we  have  a  ivater 
immersion  lens;  if  oil,  an  oil  immersion  lens.  If  the  index  of 
refraction  of  the  oil  is  about  the  same  as  that  of  glass,  we  have 
a  homogeneous  immersion  lens.  Cedar  oil  thickened  by  evap- 
oration is  an  example  of  such.  Lenses  intended  for  immersion 
must  be  constructed  accordingly.  The  great  advantage  of  im- 
mersion is  that  the  angle  of  the  cone  of  light  that  can  be  util- 
ized by  the  lens  is  considerably  increased,  thereby  increasing 
the  illumination  and  the  efficiency  of  the  microscope. 


Fig.  24  represents  the  construction  of  two  dry  objectives 
(3  and  6)  and  an  oil  immersion  objective  C^/^..  inch  focal  length) . 
Piece  3  has  %  inch  focal  length  and  consists  of  two  compound 
lenses  or  doublets,  in  which  concave  and  convex  lenses  are  com- 
bined.    Piece  6  has  one  single  lens  and  two  doublets  and  a  focal 


Oculars. 


25 


length  of  Ve  inch.  The  V12  inch  objective  has  two  single  lenses 
and  two  doublets.  The  compound  lenses  in  the  pieces  serve  the 
purpose  of  correcting  aberration  defects.  The  objectives  are 
achromatic. 

Angular  Aperture  op  a  Lens. — The  efficiency  of  an  objective 
is  in  great  part  dependent  upon  the  size  of  the  cone  of  light  it 
can  take  in  from  a  point  of  the  object  to  form  its  image.  The 
cone  of  light  utilized  is  approximately  measured  by  the  so-called 
angular  aperture.  For  a  single  lens  this  is  the  angle  formed 
by  lines  joining  the  focal  point  with  the  edges  of  the  lens.  In 
an  objective  it  is  the  angle  formed  by  the  lines  from  the  focal 
point  to  the  edges  of  the  uppermost  lens  of  the  system.  More 
accurately,  the  cone  of  light  utilized  is  measured  by  the  product 
of  the  index  of  refraction  of  the  medium  between  the  objective 
and  the  cover-glass  lying  over  the  object,  and  the  sine  of  half 
the  angular  aperture,  which  is  expressed  thus,  n.  sine  u,  where 
n  is  the  index  of  refraction  and  u  is  half  the  angular  aperture. 
This  expression  is  known  as  the  numerical  aperture.  In  the 
case  of  dry  objectives  n  is  one  (index  of  refraction  of  air)  ;  for 
water-immersion  objectives  n  is  1.33;  for  cedar-oil  immersion 
objectives  n  is  1.52.  It  will  be  seen  that  the  cone  of  light  that 
can  be  utilized  b}^  an  objective  of  a  given  angular  aperture  is 
considerably  greater  for  an  immersion  lens  than  for  a  dry  lens. 
As  the  angular  aperture  varies  inversely  as  the  focal  length,  it 
follows  that  the  shorter  the  focal  length  is  the  greater  is  the 
cone  of  light  that  the  lens  can  utilize ;  in  other  words,  lenses  of 
high  magnification  can  utilize  a  greater  cone  of  light  than  those 
of  low  magnification. 

Oculars  or  Eye-Pieces. — The  Huyghens'  eye-piece  is  univer- 
sally used.  It  is  known  as  a  yiegative  eye-piece.  Its  construc- 
tion is  shown  in  Fig.  25,  which  consists  of 
two  plano-convex  crown  lenses,  the  lower  one 
being  the  larger,  less  magnifying  (its  focal 
length  being  three  times  that  of  the  upper 
lens).  It  is  known  as  the  field  lens.  It  in- 
creases the  field  of  vision,  i.  e.,  the  number 
of  points  of  the  object  that  are  made  visible 
through  the  instrument.  Tlie  upper  lens  is 
known  as  the  eye  lens.  It  magnifies  the 
image  formed  by  the  objective.  Both  lenses 
have  their  convex  surfaces  turned  towards 
the  object.  Midway  between  them  is  a  per- 
forated diaphragm,  the  object  of  which  is  to 
cut  out  edge  rays  from  the  image  and  thus  i,i„  _5 

decrease  spherical  aberration.     The  virtues 
of  the  Huyghens'  eye-piece  are  that  it  corrects  chromatic  aber- 
ration, enlarges  the  field  of  vision  and  forms  a  flat  image,  i.  e., 


26  Vegetable  Histology. 

all  points  of  the  image  are  in  focus  at  the  same  time.  This 
latter  quality  is  essential  in  all  good  microscopes.  In  all  nega- 
tive eye-pieces  the  image  of  the  object  is  formed  between  the 
two  lenses,  and  is  then  further  magnified  by  the  eye  lens.  The 
lenses  taking  part  in  the  formation  of  the  first  image  are,  there- 
fore, the  objective  and  the  field  lens  of  the  eye-piece.  In  posi- 
tive eye-pieces  the  first  image  is  formed  below  the  field  lens, 
i-  e.,  the  ocular  takes  no  part  in  its  formation.  An  example  of 
such  is-  Ramsden's  eye-piece. 

Eye-pieces  are  designated  by  methods  like  those  stated  for 
objectives.  Those  most  commonly  used  have  focal  lengths  of 
2  inches,  II/2  inches  and  1  inch. 

There  is  a  particular  order  that  should  be  observed  in  chang- 
ing the  lenses  in  passing  from  a  low  power  to  a  high  one.  For 
example,  there  are  two  objectives,  'V3  and  ^/g  inch,  and  two  eye- 
pieces, 2  and  1  inch.  The  following  is  the  best  order  for  chang- 
ing these : 

Objective.  Eye-piece. 
2/^  inch  2  inch — Low  power. 

^/g  inch  2  inch — Medium  power. 

Vs  inch  1  inch — High  power. 

In  other  words,  it  is  better  to  increase  magnification  at  the 
objective  end  than  at  the  eye-piece  end  of  the  microscope.  The 
reason  for  this  is  that  the  eye-piece  magnifies  any  defects  of  the 
objective.  In  good  instruments  a  high  objective  is  not  likely 
to  have  more  defects  than  a  low  one,  hence  in  increasing  mag- 
nification by  changing  to  a  high  objective  the  image  will  have 
no  more  defects  than  before,  although  much  more  magnified. 

As  the  magnifying  power  is  increased  the  field  of  view  be- 
comes smaller,  illumination  of  the  image  decreases  and  the 
image  is  increased  in  size. 

Tube  Length. — Magnification  may  also  be  increased  by  draw- 
ing out  the  inner  tube,  which  increases  the  distance  between  the 
objective  and  the  plane  of  the  real  image  formed  in  the  barrel 
of  the  instrument,  and,  consequently,  the  size  of  the  image. 
The  tube  length,  however,  should  be  kept  constant,  because  the 
objectives  are  prepared  to  suit  a  definite  tube  length,  which  is 
sometimes  fixed  at  160  mm.  (6.3  inches),  sometimes  at  216  mm. 
(8V2  inches).     The  scale  on  the  inner  tube  regulates  the  length. 

Camera  Lucida. — This  is  a  drawing  ai)paratus  which  is  at- 
tached to  the  eye-piece,  and  is  used  whenever  it  is  desired  to 
make  accurate  delineations  of  the  object.  By  means  of  it,  a 
white  surface  of  paper,  on  the  table  alongside  of  the  instru- 
ment, is  reflected  into  the  eye  while  it  receives  the  image,  and 
thus  a  pencil  point  can  be  traced  on  the  paper  along  the  lines 
of  the  image,  giving  an  accurate  drawing.     The  Abbe  camera 


Requisites  op  a  Good  Microscope.  27 

is  the  best  form  in  the  market  at  the  present  time.  In  drawing, 
the  microscope  must  be  erect  and  the  paper  horizontal  and  at 
the  distance  of  distinct  vision,  about  12  inches. 

Determination  of  Magnification  in  a  Microscope. — The 
magnifying  power  for  certain  combinations  of  objectives  and 
eye-pieces  and  tube  length  is  usually  stated  by  the  makers,  so 
that  it  is  hardly  necessary  now  to  determine  the  magnifying 
power.  But  sometimes  the  rating  of  the  makers  is  not  correct 
and  we  might  want  to  use  a  different  tube  length,  and,  again, 
the  distance  of  most  distinct  vision  for  our  eyes  might  not  be 
the  average  distance,  namely,  12-14  inches,  in  which  case  a  new 
determination  must  be  made,  which  may  be  done  in  the  follow- 
ing manner : 

A  Stage  Micrometer — a  piece  of  glass,  accurately  ruled  to 
hundredths  of  a  millimeter — is  placed  on  the  stage  and  brought 
into  focus.  By  means  of  a  camera  lucida,  the  magnified  scale 
and  an  accurate  mm.  scale,  placed  at  the  distance  of  distinct 
vision  alongside  the  microscope  and  parallel  with  the  mi- 
crometer scale,  are  brought  into  superposition.  The  number  of 
mm.  divisions  covered  by  a  definite  number  of  the  micrometer 
scale  divisions  is  then  noted.  Suppose  each  magnified  scale 
division  covers  5  mm.  of  the  rule,  what  is  the  magnifying 
power?  One  mm.  is  equal  to  100  micrometer  divisions,  5  mm. 
covered  by  one  micrometer  space  are  equal  to  500  micrometer 
divisions.  Hence  one  micrometer  division  has  been  magnified 
so  as  to  cover  a  space  500  times  as  wide,  i.  e.,  it  has  been  mag- 
nified 500  times.  This,  then,  is  the  power  of  the  instrument  for 
the  particular  combination. 

Source  of  Light. — The  best  source  of  light  is  a  white  cloud 
or  the  diffused  light  reflected  from  a  white  wall  or  other  white 
object.  'Never  use  direct  sunlight.  Light  from  the  blue  sky  is 
not  so  good  as  that  from  a  white  surface.  There  is  a  tendency 
among  beginners  to  use  the  strongest  light  possible.  This  is 
injurious  to  the  eyes  and  often  obscures  details  of  the  object  by 
its  dazzling  glare.     A  window  facing  north  is  best. 

Requisites  of  a  Good  Microscope. — It  goes  without  saying 
that  the  best  workmanship  must  be  found  in  the  mechanical 
parts.  The  foot,  pillar,  arm,  stage,  etc.,  must  be  of  sufficient 
weight  and  strength  and  size.  For  the  optical  parts  five  points 
must  be  considered : 

1.  Working  Distance. — This  is  the  distance  between  the 
front  lens  of  the  objective  and  the  object.  The  lower  the  mag- 
nifying power  the  larger  the  working  distance  in  general. 
Working  distance  has  no  fixed  relation  to  the  focal  length,  but 
varies  with  the  mode  of  construction  and  the  aperture  of  the 
objective.  Of  two  objectives  having  the  same  focal  length,  that 
one  with  the  larger  working  distance  is  to  be  chosen.     As  the 


28  Vegetable  Histology. 

power  is  increased  the  working  distance  is  decreased.  It  is 
often  advantageous  to  gain  working  distance  at  the  expense  of 
magnification,  as  the  manipulation  of  objects  on  the  section 
slide  is  made  easier. 

2.  Penetrating  Power  or  Focal  Depth. — This  is  the  ver- 
tical range  through  which  the  parts  of  an  object  not  precisely 
in  the  focal  plane  may  be  seen  with  sufficient  distinctness  to 
enable  their  relations  with  what  lies  exactly  in  that  plane  to  be 
clearly  traced  out.  It  is  larger  the  smaller  the  magnifying 
power  and  numerical  aperture  are,  and  vice  versa.  Of  two 
objectives  having  the  same  power,  but  different  working  dis- 
tances, that  one  will  have  the  more  focal  depth  whose  working 
distance  is  the  greater.  It  is  often  desirable  to  see  for  a  con- 
siderable distance  into  an  object.  In  such  cases  low  power 
must  be  used. 

3.  Flatness  op  Field. — All  parts  of  the  image  must  be  in 
focus  at  the  same  time. 

4.  Defining  Power. — The  power  to  form  an  image  in  the 
highest  degree  sharply  defined,  and  free  from  color.  This 
quality  is  governed  by  the  objectives  only  and  depends  on  accu- 
rate centering  of  the  lenses  and  completenss  of  correction  for 
spherical  and  chromatic  aberrations.  Want  of  defining  power 
is  indicated  by  blurring  of  clearly-marked  lines  or  edges  and  by 
general  fog. 

5.  Resolving  Power. — By  which  very  minute  and  closely 
approximated  markings,  whether  lines,  striae,  dots  or  apertures 
can  be  separately  discerned.  This  power  varies  directly  as  the 
aperture  of  the  objective.  High  powers  have  the  greatest  re- 
solving power. 

Care  of  the  Microscope. — The  stand  should  never  be  wetted 
with  such  substances  as  alcohol,  soap,  etc.,  which  dissolve 
lacquer.  If  it  be  necessary  to  clean  the  stand,  moisten  with 
water  and  dry  with  an  old  linen  rag,  rubbing  with  the  grain 
of  the  brass.  Never  examine  objects  lying  in  acids  or  alkalies 
or  other  chemicals  without  putting  on  a  "cover"  glass.  If 
liquid  happens  to  get  on  the  objective,  rinse  off  at  once  with 
water  and  dry  with  an  old  linen  rag  or  Japanese  filter  paper. 

Be  careful  not  to  force  the  lens  down  on  the  cover  glass. 
Exercise  great  care  in  putting  objectives  and  eye-pieces  on  or 
off,  lest  they  be  dropped  and  injured. 

Directions  for  Using  the  Microscope. 

1.  The  instrument  should  be  placed  directly  in  front  of  the 
observer,  with  the  pillar  facing  backward.  Wipe  the  mirror 
with  a  soft  rag  and  turn  it  so  that  a  beam  of  light  is  thrown  up 
through  the  diaphragm.     All  ivork  should  he  hegun  with  the 


Directions  for  Using  the  Microscope.  29 

low-power  objective.  The  body  of  the  microscoi)e  should  be 
about  vertical,  so  as  not  to  interfere  with  mounting  in  fluid 
media. 

2.  The  object  mounted  on  a  glass  ^'slide"  in  a  suitable  liquid 
and  covered  with  a  "cover  glass"  is  brought  to  the  center  of  the 
diaphragm  and  focused  by  means  of  the  coarse  adjustment  in 
the  following  manner :  Using  the  left  thumb  and  forefinger  to 
adjust  the  slide,  with  the  right  hand  the  objective  is  brought 
down  so  that  it  all  but  touches  the  cover  glass,  then,  while 
looking  through  the  eye-piece,  slowly  raise  the  tube  by  the  coarse 
adjustment  until  the  object  is  in  view;  from  this  point  the 
exact  focus  can  be  made  by  turning  the  fine  adjustment  screw. 

3.  Never  lift  the  slide  from  the  stage,  but,  having  raised  the 
objective,  especially  in  case  of  high  powers,  slide  it  off'  the  stage 
without  upward  movement. 

4.  Accustom  yourself  to  use  both  eyes  indift'erently  and 
always  keep  hoth  eyes  open.  It  is  preferable  to  observe  with 
the  left  eye,  as  it  is  more  convenient  in  making  drawings. 

5.  To  mount  an  object,  place  it  in  the  center  of  a  slide  in  a 
drop  of  liquid,  say  water, ;  rest  a  cover  glass  on  its  edge  near  the 
object  in  a  slanting  position  and  gradually  lower  it  by  means 
of  a  teasing  needle  or  forceps,  in  order  to  avoid  entrapping  air 
bubbles.  The  cover  glass  should  be  previously  cleaned  with  a 
soft  rag  or  lens  paper  and  then  handled  by  the  forceps  only. 
Any  superfluous  water  on  the  slide  is  taken  up  by  a  camel's  hair 
brush  or  blotting  paper. 

6.  Cleanliness  should  characterize  all  the  work  of  the  micro- 
scopical laboratory.  All  apparatus,  slides,  cover  glasses,  etc., 
should  be  kept  scrupulously  free  from  dirt.  The  glasses  of  the 
objectives  and  eye-pieces  should  never  be  touched  with  the  fin- 
gers. Whenever  they  need  cleaning,  breathe  upon  them  and 
wipe  with  a  soft,  clean  linen  rag  or  a  piece  of  Japanese  filter 
paper. 

7.  All  objects  observed  should  be  drawn.  Drawings  are  use- 
ful, not  only  in  explaining  to  others  the  structures  observed, 
but  they  are  themselves  great  aids  also  to  accurate  observation, 
and  are  equally  helpful  in  giving  vividness  and  permanency  to 
knowledge. 

Each  student  should  provide  himself  with  a  drawing  book 
and  a  medium  pencil.  It  is  excellent  practice  to  keep  a  record 
in  writing  of  work  done  in  the  laboratory,  besides  making 
drawings. 

Some  Accessory  Apparatus  Necessary  in  Histological  Work. 

1.  Micrometer,  preferably  metric  scale.  Convenient  scale  is 
hundredths  of  a  millimeter. 

2.  Section  razor,  flat  on  one  side,  slightly  hollow  on  the 


30 


Vegetable  Histology. 


other,  for  making  thin  "sections"  or  slices  of  bodies;  also  a 
hone  and  a  strop. 

3.     A  graduated  ruler,  having  both  English  and  metric  scale. 

Dissecting  needles. 

Sharp-pointed  scissors  preferably  bent. 

Delicate  forceps  or  pincettes. 

Watch  glasses  for  holding  sections. 

Small  porcelain  evaporating  dish. 

Camel's  hair  brushes,  assorted  sizes. 

Glass  section  slides,  3"xl",  not  too  thick,  ground  edges. 

Cover  glasses,  %"  circles  No.  2. 

Camera  lucida  for  drawing. 

Polariscope.        , 

Draughtsman's  dividers,  for  drawing. 

Microtome,  for  section  cutting. 

Turn-tafc4t!-4i^a,UtHil&«Lsections. 

^ting  paper. 

be  obtained  from  any  large 

tical    Co.,   Rochester,    N.   Y. ; 

of  apparatus  may  be  seen  in 

Microscopy,  as  Behrens'  Bo- 


4. 

5. 

6. 

7. 

8. 

9. 
10. 
11. 
12. 
13. 
14. 
15. 
16. 
17. 


ABLE  HISTOLOGY 

CHAPTER    II. 

to  acqu-ire  some  familiarity  with  the  manipulation 
of  the  microscope  before  studying  vegetable  objects,  it  is  well 
to  study  some  simple  things  like  cotton,  silk,  wool  and  linen 
fibres,  and  these  are  chosen  because  they  sometimes  occur  acci- 
dentally on  the  slide  when  we  are  studying  other  things,  and 
also  because  of  their  great  practical  importance. 


Linen,  Cotton,  Silk,  Wool. 


Linen. — Scrape  a  linen  thread  on  a  glass  slide  with  a  knife 
blade  to  a  woolly  mass,  mount  a  little  of  this  on  a  slide  in  a 
drop  of  water,  taking  care  that  the  fibres  are  wetted  and  no  air 
adheres  to  them,  then  cover  with  a  cover  glass  as  described 
above.  The  student  should  guard:  against  an  error  that  begin- 
ners are  apt  to  fall  into,  namely,  putting  too  much  material  on 
the  slide.  A  very  small  quantity  will  suffice.  Examine  with 
low  power  {^/"  objective  and  2"  eye-piece).  Very  little  will 
be  made  out.     Some  clear,  smooth,  tangled  threads  will  be  seen. 


Linen,  Cotton^  Silk,  Wool.  31 

Put  on  high  power.  The  linen  will  be  seen  to  consist  of  long, 
cylindrical  fibres,  thickened  at  intervals  into  nodes,  with  a 
small  canal  looking  like  a  line  running  lengthwise  of  the  fibre. 
At  intervals  there  are  faint  cross-lines  (Fig.  26,  B).  The  walls 
are  faintly  striated  and  rather  thick,  and  the  canal,  which  is 
uniform  in  width,  may  contain  granular  remains  of  protoplasm. 
The  faint  cross-lines  become  more  prominent  when  the  fibres 
are  mounted  in  a  concentrated  aqueous  solution  of  chloral 
hydrate. 

Linen  fibres  are  of  vegetable  origin,  their  material  is  cellulose, 
a  substance  which  is  one  of  the  chief  materials  found  in  plants. 
Remove  the  cover  glass,  add  a  drop  of  a  solution  of  iodine  in 
potassium  iodide  (see  reagents),  after  a  few  minutes  render 
the  fibres  nearly  dry  by  removing  the  liquid  with  filter  paper, 
then  add  a  few  drops  of  sulphuric  acid  (see  reagents),  replace 
the  cover  glass  and  examine  again.  The  fibres  are  stained  a 
deep  blue  color  and  swollen.  This  is  a  characteristic  test  for 
cellulose  material.  Iodine  alone  does  not  color  it,  but  the  acid 
acts  on  it,  converting  it  into  a  starch-like  body  called  amyloid, 
which  stains  just  like  starch  itself  with  iodine.  Cellulose  and 
starch  belong  to  the  same  group  of  chemical  compounds,  known 
as  carb ohydrates. 

Mount  some  of  the  fibres  in  ammonio-copper  hydroxide  solu- 
tion (see  reagents)  and  note  that  they  swell  and  dissolve 
quickly,  except  a  slender  thread  from  the  center  (contents  of 
the  canal) .     The  reagent  is  one  of  the  few  solvents  for  cellulose. 

Linen  fibres  are  the  so-called  hast  fibres  found  in  the  inner 
bark  of  the  stem  of  the  Flax  plant,  Linum  usitatissimum. 

Cotton. — Mount  a  little  raw  cotton  or  non-absorbent  cotton 
wool  in  a  drop  of  alcohol ;  let  most  of  the  latter  evaporate,  then 
add  sufficient  water  and  cover  with  a  glass.  With  low  power, 
slender,  clear  fibres,  not  very  different  in  appearance  from  linen 
fibres,  will  be  seen.  Some  of  them  are  marked  by  what  appears 
to  be  constriction.  With  high  power,  long,  flat  bands,  which 
have  caved  in,  often  twisted  like  a  corkscrew,  at  times  striated 
diagonally,  will  be  seen  (Fig.  26,  C  and  D).  The  fibres  do  not 
possess  cross-lines.  They  are  the  long  hairs  on  the  seeds  of  the 
cotton  plant,  Gossypium,  the  hairs  being  plant  cells,  consisting 
at  maturity  only  of  cellulose  walls  which  fall  together,  giving 
the  fibres  the  appearance  of  a  flat  band.  The  filaments  are 
about  2  cm.  (*/.-.  inch)  long  in  short  staple  to  4  cm.  (1  Vs  inf'hi) 
long  in  long  staple  cotton,  and  about  0.02  mm.  (0.0008  inch) 
broad.  There  is  a  central  canal  running  through  each  fibre, 
much  larger  than  in  linen  fibres.  The  fibres  respond  to  tests 
for  cellulose  as  in  case  of  linen.  With  ammonio-copper  hy- 
droxide solution,  when  the  swelling  action  is  moderated,  there 
often  appear  constrictions  alternating  with  large  swellings. 
This  is  due  to  the  cuticle  which  covers  the  surface  of  the  fibres. 


32 


Vegetable  Histology. 


Linen  has  no  cuticle  and  does  not  give  the  appearance  men- 
tioned. The  cuticle  is  insoluble  in  the  reagent,  not  being  cellu- 
lose in  nature. 

Wool. — Mount  some  fibres  from  white  woolen  yarn  in  water. 
With  low  power  the  fibres  are  clear,  slightly  roughened  on  the 
surfaces  with  faint  cross-lines.  Under  high  power  the  fibres 
are  cylindrical,  containing  a  central  axial  substance,  called  the 
medulla  (not  present  in  all  hairs  or  wool).  The  surface  is  cov- 
ered by  imbricated  scales,  like  tiles  on  a  roof,  giving  to  the  edges 
of  the  fibres  a  barbed  appearance  (Fig.  26,  E).  Compare  a 
human  hair  with  wool.  If  raw  wool  be  examined,  globules  of 
fatty  matter  (wool  fat)  will  be  seen  adhering  to  the  fibres. 


Wool  being  of  animal  origin  does  not  give  the  cellulose  reac- 
tions, as  do  linen  and  cotton  fibres,  but  shows  proteid  reactions. 
Treated  with  iodine  solution  and  sulphuric  acid,  as  described 
under  linen,  the  fibres  are  stained  a  deep  yellow  and  do  not  dis- 
solve even  when  warmed.  When  warmed  on  the  slide  with  a 
saturated  aqueous  solution  of  picric  acid  the  fibres  are  stained 
yellow;  cellulose  does  not  stain  with  this  reagent.  Warmed 
with  ammonio-copper  hydroxide  solution,  wx)ol  turns  bluish- 
violet  and  its  structure  becomes  more  distinct,  but  it  does  not 
dissolve  or  swell. 

Wool  varies  in  its  details  of  structure,  and  the  identification 


Yeast.  33 

of  its  source  is  not  always  an  easy  task,  being  sometimes  well- 
nigh  impossible. 

Silk. — Scrape  some  threads,  as  in  case  of  linen,  and  mount 
in  water.  With  both  low  and  high  powers  the  fibres  appear 
about  the  same — shining,  dense,  cylindrical,  structureless,  with- 
out central  canal,  easily  distinguished  from  all  other  spun  fibres 
(Fig.  26,  A).  Silk  is  animal  in  origin  and  gives  the  same  reac- 
tions as  wool.  Several  other  bast  fibres  are  very  valuable  in 
textile  industries  and  present  appearances  similar  to  that  of 
linen. 

Hemp  is  derived  from  Cannabis  sativa,  jute  from  Corchorus 
olitorius  and  Corchorus  capsularis,  Manila  hemp  from  Musa 
textilis.  More  detailed  study  of  various  fibres  may  be  found 
in  some  large  work,  for  example,  Die  Mikroskopie  der  technisch- 
verwendeten  Faserstoffe,  by  von  Hohnel. 


CHAPTER    III. 
Yeast  (Torula  or  Saccharomyces  cerevisi^). 

This  is  a  plant  and  is  that  which  causes  alcoholic  fermenta- 
tion in  sugar  solutions.  It  is  a  plant  of  the  simplest  kind,  con- 
sisting of  a  single  cell.  Plants  are  divided  into  four  series 
according  to  their  complexity  of  structure  and  functions. 

Thallophyta-(Thallus  plants)  1  Cryptogamia  or  Flowerless 

Bryophyta— (Moss  plants)  V      plants. 

Pteridophyta — (Fern  plants)     J 

Spermaphyta  or  Phanerogamia  or  Flov/ering  plants. 

The  Thallophyta  are  a  large  group  of  plants  in  which  there 
is  no  clear  differentiation  of  the  plant  body  into  root,  stem  and 
leaf.  A  vast  number  of  forms  are  included,  which  differ  greatly 
among  themselves  in  complexity,  but  even  the  highest  forms 
never  have  true  roots,  and  in  the  great  majority  of  cases  there 
is  no  differentiation  into  stem  and  leaves.  There  is  never  a 
clear  differentiation  into  epidermal,  fundamental  and  fibro-vas- 
cular  systems  of  tissues  as  in  the  ferns  and  flowering  plants. 

The  Thallophyta  are  divided  into  a  number  of  classes,  one  of 
which  is  called  Fungi,  or  Moulds.  It  is  thought  by  some  that 
yeast  belongs  to  the  fungi,  being  a  degenerate  form.  Yeast 
occurs  both  wild  as  well  as  cultivated,  the  former  living  upon 
fruits  or  in  fruit  juices  and  occurring  in  the  air,  the  latter  be- 
ing employed  in  brewing  and  for  making  bread,  etc.  There  are 
a  number  of  species  of  wild  and  cultivated  yeasts,  and  it  is  prob- 
able that  cultivated  yeasts  are  descended  from  similar  forms 
of  wild  yeasts.     Saccharomyces  cerevisise,  or  brewers'  yeast,  is 


34  Vegetable  Histology. 

one  species,  but  ordinary  commercial  yeast  seldom  consists  of 
this  species  alone. 

Sow  some  fresh  baker's  yeast  in  Pasteur's  fluid  and  keep  in  a 
warm  place.  As  soon  as  the  solution  begins  to  froth  and  the 
yeast  is  manifestly  increasing  in  quantity  it  is  ready  for  study. 
Fermentation  is  most  active  between  28°  and  34°  C.  At 
38°  C.  growth  ceases. 

Mount  a  drop  of  the  liquid  and  examine  with  low  power, 
minute  specks  will  be  seen.  With  high  power  numerous 
rounded  or  ellipsoidal  bodies  will  be  seen,  either  single  or  loosely 
united  into  short  chains.  The  diameter  of  the  cells  varies  from 
V2500  to  ^Aooo  i^ch  (average,  V3000  inch).  Each  torula  con- 
sists of  a  well-defined  homogeneous  transparent  sac  or  cell-wall 
of  cellulose  material,  enclosing  a  semi-fluid  granular  substance 
called  protoplasm^  within  which  there  is  often  a  space  full  of 
a  more  watery  fluid  than  the  rest,  termed  a  vacuole.  The  whole 
structure  is  known  as  a  cell.  The  cell-wall  is  comparatively 
tough,  but  may  easily  be  burst  and  the  contents  thrown  out;  it 
is  thicker  in  old  cells  than  in  young  actively  growing  ones. 
Minute  shining  dots,  thought  to  be  fat  globules,  may  be  seen 
in  the  protoplasm,  but  there  is  neither  chlorophyll  nor  starch 
present.  By  the  use  of  special  reagents,  a  nucleus  has  been 
demonstrated  to  be  present  in  the  cells,  but  it  is  never  seen  in 
the  living  cells. 

Torulse  break  down  sugar  mainly  into  alcohol  and  carbon 
dioxide  gas,  and  at  the  same  time  increase  in  number.  Multi- 
plication takes  place  in  this  way.  A 
small  protuberance  begins  to  form  on 
the  parent  torula,  which  grows  larger, 
forming  a  bud.  The  bud  increases  un- 
til it  attains  the  size  of  the  parent 
torula  and  eventually  becomes  detach- 
ed, though  generally  not  until  it  has 
developed  other  buds  on  itself  and  these 
still  others.  The  torulse  produced  thus 
by  gemmation  or  budding  are  apt  to  ^^^*  ^^* 

adhere  to  each  other  for  a  long  time  and  thus  produce  heaps 
and  strings  (Fig.  27). 

Mount  a  drop  of  the  yeast  culture  in  a  small  drop  of  fuchsin 
stain.  Note  which  cells  stain  most  rapidly  and  deeply. 
Actively-growing  protoplasm  is  not  stained  readily  by  many 
dyes,  while  dead  or  passive  protoplasm  is  colored  quickly  by  the 
same  dyes.  The  cell-wall  is  unaffected  and  the  vacuole  also, 
although  the  latter  may  appear  purplish,  because  it  is  seen 
through  a  colored  layer  of  protoplasm. 

Make  another  mount  and  apply  iodine  solution  at  the  edge  of 
the  cover  glass.     As  the  iodine  diffuses  under  the  glass  the 


Yeast.  35 

protoplasm  of  the  cells  is  stained  yellowish-brown.  This  is 
one  of  the  best  tests  for  proteid  matter.  The  absence  of  blue- 
stained  particles  is  proof  that  the  cells  contain  no  starch.  The 
cell-wall  is  not  stained. 

Yeast  occurs  in  the  market  in  a  dry  or  pasty  condition  as 
"yeast  cakes."  Make  an  emulsion  of  a  bit  of  one  of  these  cakes 
in  water  and  examine  a  drop  under  high  power.  Cells  similar 
or  identical  to  those  seen  in  the  yeast  culture  are  in  abundance. 
In  the  pasty  cake  there  is  also  present  rounded  bodies  many 
times  larger  than  the  yeast  cells.  These  are  starch  grains, 
usually  of  potato  starch,  which  are  added  to  absorb  the  water 
of  the  mass  of  yeast  cells  in  order  to  render  it  semi-solid  and 
capable  of  being  moulded  into  cakes. 

In  brewing  industries  two  well-defined  varieties  of  yeast  are 
used,  known  respectively  as  top  and  bottom  yeast.  Top  yeast 
is  employed  in  making  English  ale,  stout  and  porter,  fermenta- 
tion taking  place  at  ordinary  summer  temperature  and  pro- 
ducing carbon  dioxide  rapidly  enough  to  cause  the  yeast  to 
collect  at  the  surface  of  the  liquid,  hence  the  name  top  yeast. 
Bottom  yeast  is  used  in  making  "lager"  beer,  and  grows  quietly 
at  the  bottom  of  the  vat  at  a  temperature  of  about  4°  C,  which 
is  kept  constant  by  artificial  means.  Besides  this  difference  in 
conditions  of  growth,  the  two  yeasts  also  differ  in  form,  size  and 
structure  when  seen  under  the  microscope.- 

Torula  is  classed  among  the  plants,  because  it  has  a  cellulose 
cell-wall  and  the  power  of  constructing  protoplasm  (living 
matter)  out  of  comparatively  simple  substances,  such  as  am- 
monium tartrate,  which  is  distinctively  a  vegetable  peculiarity. 
But  though  a  plant,  it  contains  neither  starch  nor  chlorophyll, 
and  cannot  obtain  the  whole  of  its  food  from  inorganic  com- 
pounds, thus  differing  widely  from  green  plants. 

Pasteur^s  Solution. — Potassium  phosphate,  2  parts;  cal- 
cium phosphate,  0.2  parts;  magnesium  sulphate,  0.2  parts;  am- 
monium tartrate,  10  parts;  cane  sugar,  150  parts;  water,  838 
parts. 


36  Vegetable  Histology. 

CHAPTER    IV. 

Bacteria  (Schizomycetes^  or  Fission  Moulds). 

One  of  the  classes  of  the  Thallophyte  series  of  plants  is  the 
Schizophyta.  This  class  is  composed  chiefly  of  the  Schizomy- 
cetes  or  Bacteria.  These  are  extremely  low  forms  of  plant  life, 
being  exceedingly  simple  in  structure  and  always  minute,  some 
of  them  being  the  smallest  of  known  organisms.  They  are 
mostly  unicellular,  or,  if  consisting  of  cell-aggregates,  as  is 
sometimes  the  case,  the  cells  are  united  in  a  simple  way  and 
have  very  little  dependence  upon  each  other.  They  are  the 
most  abundant  of  organisms,  the  largest  being  not  more  than 
^/loooo  iiich  in  diameter  and  the  smallest  not  more  than  ^/^o  of 
that.  All  are  chlorophylless,  i.  e.,  without  coloring  matter. 
The  cells  agree  in  having  mostly  rigid  transparent  walls  and 
colorless  cell-contents,  but  different  species  differ  considerably 
in  form,  size,  etc.  Their  usual  mode  of  increase  is  by  fission 
or  splitting,  but  they  also  produce  very  minute  so-called  spores 
by  a  method  known  as  internal  eell-formation.     (See  later.) 

In  some  species  the  cells,  after  fission,  immediately  become 
independent;  in  others  they  remain  united  for  a  time,  to  form 
filaments  or  chains  of  various  lengths.  Many  of  the  species  in 
some  stage  of  their  development  have  the  habit  of  secreting  a 
jelly  and  increasing  rapidly  by  fission,  forming  large  gelatinous 
colonies.  These  are  called  zoo glcea-m asses.  ''Mother-of-vin- 
egar"  and  the  so-called  "blood-rain,'^  consisting  of  red  gelatin- 
ous spots  often  found  on  putrefying  bread,  are  examples  of 
zoogloea-masses. 

In  all  putrefying  fluids  or  solutions  that  contain  decaying 
organic  matter  bacteria  swarm  in  myriads.  They  are,  in  fact, 
the  inciting  cause  of  putrefaction.  By  their  agency  also  milk 
sours,  wine  is  converted  into  vinegar,  etc.  So  far  as  animal  life 
is  concerned,  some  of  the  species  are  harmless  or  perhaps  even 
beneficial,  while  others  are  the  source  of  some  of  the  most 
dreaded  and  most  fatal  of  diseases.  Chicken  cholera,  splenic 
fever,  typhoid  fever,  diphtheria  and  leprosy  are  examples.  A 
peculiar  interest  therefore  attaches  to  the  study  of  these 
organisms. 

Bacteria  are  killed  at  about  70°  C.  or  above,  but  the  spores 
can,  in  many  cases,  survive  a  temperature  above  100°  C. 
(Spores  are  little  specialized  cells  which  have  the  power  of 
developing  into  cells  or  plants  in  all  respects  like  the  ones  from 
which  they  were  derived.  In  the  case  of  bacteria  they  are 
formed  for  protective  and  not  for  reproductive  purposes;  they 
can  withstand  conditions  of  temperature,  etc.,  under  which  the 
ordinary  cells  die  and  thus  ensure  the  survival  of  the  bacteria.) 

Bacteria  are  conveniently  grouped  according  to  their  shapes. 


Bacteria.  37 

as  illustrated  in  Fig.  28,  in  which  a  is  the  Micrococcus  or  spher- 
ical form ;  b  the  Bacterium  or  rod-like  .form ;  c  the  Bacillus  or 
filiform  form,  and  d  the  Spirillum  or  coiled  form.  In  growing, 
bacteria  are  often  grouped  in  different  arrangements,  to  which 
special  names  are  applied,  thus  Streptococcus^  a  moniliform  or 
necklace-like  grouping  of  cocci;  Diplococcus,  cocci  in  pairs; 
Staphylococcus^  single  cocci ;  Leptothrix,  a  filament  of  bacilli ; 
Sarcina,  a  plate  of  cocci. 


O    o  O  ^  Q 

00 


o<» 


9  m  (^^ 


Fig.  28, 

Make  an  infusion  of  fresh  hay  by  steeping  it  in  water  warmed 
to  between  40°  and  50°  C.  for  one-half  hour  or  more,  filter  and 
set  aside  for  36  hours  or  more.  The  liquid  becomes  cloudy,  due 
to  swarms  of  bacteria.  Examine  a  drop  of  the  liquid  under  the 
highest  power  at  command.  Focusing  must  be  done  very  care- 
fully because  of  the  extreme  minuteness  and  transparency  of 
the  cells.  The  illumination  must  be  somewhat  dimmed,  else 
the  bacteria  will  be  almost  invisible  in  the  glare.  A  conveni- 
ent way  to  find  the  focal  plane  is  to  move  the  slide  about  until 
a  coarser  object,  like  a  speck  of  dust,  comes  into  view,  and  with 
this  as  a  guide  to  make  the  fine  adjustment  of  the  focus. 

Note  the  moving  bacteria,  elliptical  or  rod-like,  sometimes 
forming  short,  jointed  rows.  The  cells  have  an  outer,  more 
transparent  wall,  enveloping  a  more  opaque  substance. 

Apply  a  drop  of  iodine  solution  at  the  edge  of  the  cover  glass. 
The  bacteria  are  killed,  all  forward  motion  ceases,  the  con- 
tents of  the  cells  are  stained  and  become  more  conspicuous. 
The  cell-wall  does  not  stain.  Other  forms  that  may  be  found 
are  micrococcus,  bacillus  and  spirillum. 

Resting  Bacteria  or  Zooglma  Stage. — The  hay-infusion  after 
a  time  develops  a  scum  or  zoogloea.  Mount  a  little  of  this 
and  examine  with  high  power.  Myriads  of  bacteria  resting  in 
a  gelatinous  mass  will  be  seen.  Although  they  do  not  move 
away  from  their  places,  the  bacteria  will  be  seen  to  have  a 
wiggling  or  oscillatory  motion,  known  as  the  Brownian  move- 
ment. This  motion  is  not  a  vital  one,  but  is  characteristic  of 
very  small  bodies,  whether  dead  or  alive.  Fine  clay,  pumice, 
lamp-black,  gamboge,  show  the  same  motion.  The  cause  is  not 
definitely  known. 

Raise  the  cover  glass  and  add  a  drop  of  iodine  solution  to  the 
scum.  The  bacteria  stain,  but  not  the  gelatinous  material  in 
which  they  are  imbedded. 

Mount  a  little  of  the  scum  composing  "mother-of-vinegar." 
Nearly  the  same  appearance  will  be  seen  as  in  the  case  of  the 
scum  of  the  hay-infusion.     The  scum  is  known  as  mycoderma 


38 


Vegetable  Histology. 


aceti^  and  the  bacteria  cause  the  oxidation  of  dilute  alcohol  to 
acetic  acid  (vinegar). 

In  order  to  show  the  position  of  the  plants  already  studied, 
as  well  as  those  to  follow,  in  the  system  of  classification  of 
plants,  a  table  of  the  Thallophyta,  with  the  subdivisions,  is  here 
appended. 

Thallophytb  Series. 


Class. 

1.  Myxomycetes. 

2.  Schizophyta. 


3.  Algae. 


4.  Fungi. 


Sub-class. 

1.  Schizomycetes. 

(Bacteria.) 

2.  Cyanopliycese. 

1.  Diatomacese. 

2.  Chloropliycese. 


3. 
4. 
1. 
2. 

3.  Pliycomycetes. 


4.  Ascomycetes. 


Order. 


Oenus. 


1. 

2. 

3. 

4. 

5.  Conjugatae. 

6. 


1. 

2.  Spirogyra. 

3.  Zygnema. 
4. 


1.  Zygomycetes  (Mucor  mould). 
2. 

3. 

1. 

2.  Erysiphese  (Penicillium  mould). 
3. 

4.  Pyrenomycetes  (Ergot  of  rye). 

5. 

G.  Saccharomycetes  (Yeast). 


1. 

6.  Basidiomycetes.  2- 

3.  Hymenemycetes. 


1. 
2. 
3. 
4. 

5.  Agaricinese 
(Mushrooms). 


5.  Lichenes. 


The  table  is  incomplete,  only  those  sub-divisions  being  given 
with  which  the  plants  studied  in  these  lessons  are  concerned. 


Spirogyra.  39 

CHAPTER    V. 
Spirogyra. 

This  plant  belongs  to  the  third  class  of  the  Thallophyte 
series,  which  is  known  as  Algce.  The  class  includes  nearly  all 
the  Thallophyte  plants  which  contain  chlorophyll. 

The  algae  are  an  assemblage  of  quite  simple  plants,  none  of 
the  members  attaining  any  great  degree  of  complexity.  For 
the  most  part,  the  plant  body  consists  of  an  elongated  filament 
composed  of  united  cells;  sometimes,  however,  they  form  sur- 
faces,'and  in  other  cases  the  plants  are  unicellular  or  aggre- 
gated into  communities.  In  these  plants  we  find  the  first 
examples  of  undoubted  sexuality,  and  throughout  the  group 
the  organs  and  methods  of  fertilization  are  nearly  enough  uni- 
form to  enable  us  to  use  them  as  distinguishing  characters. 

The  algae  are  for  the  most  part  aquatic  plants  and  inhabit 
either  fresh  or  salt  water.  They  abound  in  ponds  and  slow- 
running  streams. 

Spirogyra  will  illustrate  the  characteristics  of  the  class 
(Fig.  29).  Its  position  in  the  system  of  plants  is  given  in  the 
table  (Chapter  IV).  It  belongs  to  the  order  Conjugatae.  This 
order  differs  from  all  other  algae  in  the  peculiarly  complex 
structure  of  the  chlorophyll  bodies  and  the  mode  of  sexual 
reproduction  (except  some  of  the  Diatomaceae)  which  consists 
in  the  direct  conjugation  or  union  of  two  ordinary  vegetative 
cells,  hence  the  name  Conjugatce.  Spirogyra  is  a  filamentous 
plant,  very  common  in  ponds  and  ditches  as  a  green  scum  com- 
posed of  silky,  green  threads,  which  sometimes  attain  a  length 
of  six  or  eight  inches.  The  filaments  are  unbranched  and  com- 
posed of  a  row  of  cylindrical  cells  all  alike  and  independent  of 
each  other  and  loosely  joined  together.  The  name  is  given  in 
allusion  to  the  fact  that  the  chlorox)hyll  bodies,  i.  e.,  the  bodies 
bearing  the  green  coloring  matter,  form  spiral  bands  winding 
around  the  cell  on  the  interior  of  the  cell-wall.  Sometimes  the 
bands  are  single,  at  other  times  double  or  treble  (Zygnemas 
have  stellate  chlorophyll  bodies,  two  in  each  cell,  arranged 
axially).  At  intervals  along  each  band  are  to  be  seen  highly 
refractive  lenticular  bodies  called  pijrenoids.  When  exposed 
to  the  light  for  some  time  the  pyrenoids  would  be  found,  on 
appropriate  treatment,  to  be  surrounded  by  starch  grains. 

The  cells  are  bounded  by  well-marked,  refractive  cellulose 


40 


Vegetable  Histology. 


walls.  Next  to  the  wall  is  a  thin  layer  of  protoplasm,  better 
seen  by  staining  with  iodine  solution. 
The  chlorophyll  bands  are  in  contact 
with  this  layer  of  protoplasm.  The 
greater  part  of  the  interior  of  the  cell 
is  occupied  by  a  large  vacuole  contain- 
ing cell  sapy  i.  e.,  water  with  sub- 
stances in  solution.  Each  cell  has  a 
usually  centrally-placed,  distinct  pro- 
toplasmic body  known  as  a  nucleus^ 
with  radiating  extensions  of  proto- 
plasm passing  from  it  to  the  outer 
layer  of  protoplasm  next  the  cell-wall. 
The  growth  of  spirogyra  in  length  is 
brought  about  by  cell  division.  Each 
cell  is  repeatedly  divided  into  two  equal 
parts  by  the  appearance  in  it  of  a 
cross-partition.  This  process  takes 
place  during  the  night,  and  special 
precaution  must  be  taken  in  order  to 
study  it.  This  method  of  cell  forma- 
tion is  the  general  mode  throughout 
the  vegetable  kindom. 

The  method  of  reproduction  in 
spirogyra  is  a  sexual  one  and  known 
as  conjugation.  This  process  occurs 
from  early  spring  to  June  and  July, 
but  can  be  induced  when  the  plant  is 
under  cultivation  by  allowing  the 
water  in  which  it  is  growing  to  slowly  evaporate.  Two  fila- 
ments arrange  themselves  side  by  side,  and  the  cells  lying  oppo- 
site each  other  send  out  each  a  process  or  tube ;  these  unite  and 
the  protoplasm  from  one  cell  passes  over  and  coalesces  with 
that  in  the  cell  opposite.  In  Fig.  29  two  such  tubes  about  to 
unite  are  shown  at  b,  while  the  beginning  of  formation  of  two 
other  tubes  is  shown  at  a. 

The  result  of  the  process  is  a  new  cell  called  a  zygospore. 
This  is  set  free  by  decay  of  the  walls  of  the  old  cell  and  falls  to 
the  bottom  of  the  water  and  rests  until  proper  time  for  growth. 

It  is  not  an  easy  matter  to  find  conjugating  forms  of  spi- 
rogyra, and  their  study  is  not  well  suited  for  class  work. 

There  are  a  number  of  species  of  spirogyra,  and  the  student 
should  keep  a  lookout  for  different  kinds  of  filaments  in  the 
specimen  studied.  Mount  some  filaments  in  water  and,  under 
low  power,  note  the  great  length  of  the  filaments,  as  well  as  of 
the  individual  cells,  the  uniform  diameter,  the  well-defined  cell- 
walls  and  the  conspicuous  green  chlorophyll  bands,  the  shape 
ind  relative  lensrth  and  breadth  of  the  cells. 


Fig.  29.— Spirogyra  longata 
(Bessey). 


Reproduction.  41 

With  high  power,  note  more  especially  the  crenulated  and 
wrinkled  margin  of  the  bands  and  the  refractive  nodules  at 
intervals  along  them  (pyrenoids).  Look  for  a  nucleus.  This 
is  sometimes  hidden  by  the  chlorophyll  bands  and  then  not 
easily  seen,  but  there  will  always  be  found  some  cells  in  which 
the  nucleus  stands  out  clearly. 

Apply  iodine  solution.  The  bands  stain  deeply,  especially 
the  dense  pyrenoids,  which  appear  almost  black.  The  nucleus 
also  appears  much  more  conspicuous  when  stained. 

It  may  not  be  possible  to  obtain  sprirogyra  in  a  growing  state 
at  the  time  it  is  wanted  for  study.  In  such  a  case  it  should  be 
collected  at  some  other  time,  when  a  supply  is  found,  and  pre- 
served for  future  use  in  dilute  formaldehdye  solution  (one  vol- 
ume of  40  per  cent,  ^^formalin"  to  10  or  15  volumes  of  water). 
In  this  solution  the  color  is  bleached  only  slowly,  and  if  the 
specimen  has  not  been  kept  too  long  it  will  still  show  a  green 
color. 

Another  large-sized  alga  often  found  in  slow  streams  is  the 
Hydrodictyon  or  Water-net.  The  cells  are  united  into  network, 
which  has  the  shape  of  an  elongated  bag,  often  8  or  10  inches 
long.  The  cells  are  quite  large,  cylindrical  and  filled  with 
dense  granular  chlorophyll  matter.  The  ends  of  three  or  four 
cells  meet  at  a  common  point.  Several  nets  may  be  entangled 
in  one  another,  representing  different  generations,  the  cells  of 
the  younger  nets  being  much  smaller  than  those  of  the  old  and 
matured  nets. 

The  structure  is  best  studied  under  low  objective  in  a  watch 
glass  containing  water,  as  thus  crushing  of  the  net  is  avoided, 
which  would  happen  under  a  cover  glass. 


CHAPTER    VI. 

Reproduction. 

This  is  the  power  that  plants  possess  of  giving  rise  to  new 
individuals,  and  the  process  takes  place  by  one  of  three  ways, 
namely.  Division,  Rejuvenescence  and  Union.  The  first  two 
modes  are  asexual,  the  last  sexual. 

There  are  three  varieties  of  reproduction  by  division  : 
[Fission. 
Division.^  Gemmation. 

[internal  cell  formation. 
Fission. — The  most  common  mode  of  division.     This  is  the 
separation  of  a  cell  into  equal  portions. 

a.  A  constriction  takes  place  in  the  middle  of  the  cell  and 
along  the  plane  of  this  constriction;  the  cell-walls  may  grow 


42  Vegetable  Histology. 

inward  until  the  cell  contents  become  separated  into  two  equal 
portions.  This  mode  has  been  observed  in  some  of  the  lower 
algse  (Spirogyra). 

6.  A  delicate  partition  of  cellulose  may  at  once  be  formed 
through  the  middle  of  the  cell.  This  is  the  usual  mode  by 
which  ^^tissues"  are  formed  and  growth  takes  place  in  all  the 
higher  plants. 

Gemmation. — This  method  is  found  in  the  yeast  plant  and 
its  relations.     (See  Torula  for  description.) 

Internal  Cell  Formation. — The  protoplasm  of  a  cell  breaks 
up  into  two  or  more  rounded  masses,  each  of  which  eventually 
acquires  a  cell-wall  of  its  own  and  escapes  from  the  parent  cell 
by  the  rupture  or  decay  of  the  old  cell-wall.  Examjjle,  asco- 
spores  in  lichens  and  some  fungi  and  pollen  grains  in  the 
anthers  of  flowering  plants. 

Rejuvenescence. — The  protoplasm  aggregates  into  a  rounded 
mass,  escapes  through  the  cell-wall  and  subsequently  forms  a 
new  cell-wall.  Commonly,  before  the  new  cell-wall  forms,  the 
protoplasm  forms  cilia  and  moves  about.  Rejuvenescence  is 
found  only  among  lower  forms  of  plant  life;  for  example, 
(Edogonium,  one  of  the  algse. 

As  was  said  above,  Division  and  Rejuvenescence  constitute 
asexual  reproduction.  There  are  two  modes  of  asexual  repro- 
duction : 

1.  Vegetative  reproduction. 

2.  Spore  reproduction. 

In  the  former  the  parent  plant  throws  off  from  itself  ordinary 
vegetative  cells;  in  the  latter,  specialized  cells  called  spores 
are  formed.  Examples  of  the  first  are  bacteria,  cell  multipli- 
cation in  higher  plants,  multiplication  in  case  of  many  plants 
by  bulbs,  tubers,  stolons,  offsets,  etc. 

Spore  reproduction  by  the  asexual  process  is  exemplified  in 
many  flowerless  plants.  Examples,  spores  on  the  gills  of  the 
common  mushroom,  motile  spores  so  commonly  produced  by 
the  mosses  and  ferns.  Spores  are  commonly  borne  in  a  special 
organ  called  a  sporangium. 

Union  op  Cells. — Sexual  Reproduction. 

This  consists  in  the  coming  together  and  blending  of  the  pro- 
toplasm of  two  distinct  cells  to  form  a  new  one. 

a.  The  uniting  cells  may  be  alike  and  the  process  is  then 
known  as  conjugation^  and  is  found  only  in  certain  low  forms 
of  plant  life,  as  Mucor  (a  mould),  Diatoms,  Spirogyra,  Des- 
mids,  all  of  which,  except  the  first,  belong  to  algse. 

h.    The  uniting  cells  may  be  unlike,  the  process  being  then 


Moulds.  43 

known  as  fertilization.  One  cell  (the  male  or  sperm  cell)  is 
commonly  not  only  smaller,  but  more  active  than  the  other 
(called  the  female  or  germ  cell).    Example,  all  higher  plants. 


CHAPTER   VII. 

Moulds  (Fungi). 

Moulds  belong  to  the  class  of  plants  known  as  Fungi,  which 
latter,  as  we  have  already  seen,  form  one  of  the  divisions  of  the 
Thallophyta.  The  fungi  are,  in  their  habits,  chlorophylless 
saprophytes  or  parasites.  (A  saprophyte  is  a  plant  which  de- 
rives its  sustenance  from  decaying  organic  matter.  A  parasite 
lives  on  other  organisms.)  In  all  but  a  few  instances  (see 
Torula),  their  vegetative  parts  consist  of  slender  segmented  or 
unsegmented,  usually  colorless  filaments,  each  one  being  known 
as  a  hypha.  These  ramify  among  decaying  organic  debris  or 
invade  the  tissues  of  living  organisms,  plant  or  animal,  and 
derive  their  sustenance  from  them.  In  the  simpler  hyphal 
forms  the  hyphae  occur  singly  or  more  or  less  interwoven  into  a 
tangled  felt-work,  but  they  are  not  gathered  into  definite  forms 
and  have  little  or  no  dependence  on  each  other.  In  the 
higher  groups,  however,  there  is  more  or  less  division  of  labor 
among  the  hyphae,  and  they  become  consolidated  into  false  tis- 
sues, which  acquire  definite  shapes  according  to  the  species. 
Of  this  character  are  the  fructifying  organs  or  carpophores, 
which  constitute  the  above-ground  parts  of  the  agarics,  puff- 
balls,  cup-fungi,  etc.,  and  the  sclerotium,  a  compact  hard  mass 
of  thick-walled  hyphae,  which  serves  as  a  resting  stage  in  the 
development  of  some  species,  for  example.  Ergot  of  rye. 

Fungi  reproduce  asexually  by  means  of  spores,  known  as 
gonidia  or  conidia.  These  are,  as  a  rule,  thick-walled  cells, 
which  become  separated  from  the  parent  hyphae  in  ways  which 
are  more  or  less  characteristic  in  the  different  groups.  In  all 
hyphal  fungi  the  hyphae  consist  of  two  portions — the  vegetative, 
which  ramifies  in  the  substratum,  often  forming  tangled,  felt- 
like masses  of  threads  called  the  mycelium;  and  the  repro- 
ductive^ which  comes  to  the  surface.  The  latter  produces  the 
conidia,  which  may  be  borne  on  isolated  filaments,  as  in  the 
bread-mould  (Penicillium)  or  on  a  carpophore,  which  produces 
a  spore-bearing  hymenium.  The  common  mushroom  (Agaricus 
campestris)  is  an  example  of  the  latter,  the  plate-like  bodies 
or  gills  on  the  under  surface  of  the  cap  constituting  the 
hymenium. 

'  In  a  large  number  of  fungi,  including  some  of  the  most  highly 
organized  forms,  sexual  reproduction  is  unknown.     In  other 


u 


Vegetable  Histology. 


species  sexual  reproduction  takes  place,  and  this  may  be  by 
several  methods,  namely,  conjugation,  and  formation  of  so- 
called  oospores. 

Penicillium  Glaucum   (Bread  Mould). 

This  mould  is  familiar  to  everyone  from  its  forming  sage- 
green  crusts  upon  bread,  jam,  old  boots,  etc.  It  may  be  ob- 
tained at  any  time  by  placing  a  moist  piece  of  bread  under  a 
bell-jar  in  a  moderately  warm  place.  When  spores  appear, 
sow  some  in  Pasteur's  fluid  (see  under  Torula).  Moulds  grow- 
ing in  this  fluid  are  easier  to  examine  than  when  growing  on 
bread.  On  examining  a  patch  of  mould  on  the  surface  of  the 
fluid,  it  is  found  to  consist  of  a  horizontal  felt  work  of  delicate 
tubular  filaments,  the  hyphse,  forming  a  crust  like  so  much 
blotting  paper,  which  is  known  as  the  mycelium.  Hyphae  pro- 
ject from  this  into  the  air,  b,  and  bear  a  green  powder,  the 
spores,  c.  Fig.  30.  These  hyphae  are  called  aerial.  From  the 
mycelium  other  hyphae  grow  down  into  the  liquid  and  are  called 
submerged  hyphw^  corresponding  somewhat  to  the  roots  of 
higher  plants. 


Fig.  30.— Penicillium  glaucum. 

Carefully  make  a  thin  section  of  the  mycelium  by  cutting 
between  two  pieces  of  pith.  Moisten  a  section  on  a  slide  with 
alcohol,  allow  the  latter  to  nearly  evaporate,  then  add  water 
and  cover.  Examine  first  under  low  power  and  observe  the 
appearances  described  above. 

With  high  power,  observe  that  each  hypha  has  a  transparent 
wall  and  protoplasmic  contents  and  is  divided  by  transverse 
partitions  into  a  number  of  cells.  Each  cell  has  several  large 
clear  spaces,  the  vacuoles  and  a  number  of  nuclei  which,  how- 
ever, are  only  visible  by  staining  properly. 


MucoR  Stolonifer.  45 

The  hyphse  frequently  branch  and  are  inextricably  entagled 
with  one  another,  but  every  hypha  with  its  branches  is  quite 
distinct  from  every  rather  one.  If  the  section  be  a  little  too 
thick  and  obscure,  gentle  tapping  on  the  cover  glass  over  the 
section  will  spread  the  parts,  so  that  they  may  be  more  easily 
seen. 

Note  the  aerial  hyphse,  with  brushes  ur  branches,  which  be- 
come constricted  on  their  ends  into  a  series  of  rounded  spores 
like  a  row  of  beads.  These  hyphae  which  bear  the  spores  or 
conidia  are  called  conidiaphores.  The  conidia  form  the  loose 
green  powder  characteristic  of  the  mould.  The  spore  is  a 
round,  transparent  sac  enclosing  a  mass  of  protoplasm  and  is 
in  all  essential  respects  similar  to  a  torula.  When  sown  in  an 
appropriate  medium  (Pasteur's  solution)  it  germinates  and 
forms  hyphse  from  several  points,  forming  a  new  plant  like  the 
original  one  (Fig.  30,  d). 

Stain  different  sections  with  f uchsin,  haematoxylin  and  iodine 
and  note  results. 

Besides  Penicillium,  usually  other  moulds  will  be  found  on 
mouldy  bread  or  other  matters.  The  most  prominent  among 
them  will  probably  be  Eurotium  Aspergillus  glaucus,  which 
may  be  distinguished  from  Penicillium  by  its  higher  growth, 
less  velvet-like  appearance  and  the  olive-green  color  of  the 
spores.  The  conidiaphores  of  Eurotium  are  about  ^/ig  inch 
long,  visible  to  the  eye,  and  bear  roundish  white  (unripe)  or 
pale-green  heads  closely  packed. 

A  pure  growth  of  either  of  the  fungi  described  above  may  be 
obtained  as  follows:  Place  a  few  thoroughly  boiled  (and  thus 
sterilized)  French  plums  on  a  sterile  glass  plate  and  infect  them 
with  spores  taken  from  as  pure  a  patch  of  the  mould  as  can  be 
found  by  a  previously  heated  and  cooled  needle.  Cover  the 
plate  with  a  sterile  bell  glass  and  keep  in  a  moderately  warm 
place.  (The  plate  and  bell  glass  may  be  sterilized  by  placing 
them  in  an  air-bath  heated  to  105°  or  110°  C.  for  one-half  hour 
or  more). 

MucoR  Stolonifer. 

This  mould  may  be  grown  by  keeping  some  bread  very  moist 
and  warm  under  a  bell  jar  or  by  placing  some  moist  Poke-root  in 
a  bottle  and  closing.  Mucor  is  similar  to  Penicillium  in  its 
growth,  consisting  of  a  mycelium  from  which  grow  erect  or 
aerial  hyphse,  each  one  bearing  a  rounded,  dark  head  or  spore 
case,  looking  like  a  pin  head  and  called  a  sporangium.  The 
w^all  of  the  spore  case  is  beset  with  minute  asperities  of  oxalate 
of  lime,  and  inside  the  case  are  a  great  number  of  minute  oval 
bodies,  the  spores,  held  together  by  a  transparent  intermediate 
substance.  When  ripe,  the  thin  and  brittle  coat  of  the  case 
bursts  at  the  slightest  pressure,  setting  free  the  spores.     A  lit- 


46 


Vegetable  Histology. 


tie  portion  of  the  wall  of  the  spore  case  remains  adhering  to 
the  stalk  as  a  collar.  The  cavity  of  the  stalk  does  not  com- 
municate with  the  sporangium,  but  is  cut  off  by  a  bulging  par- 
tition, forming  a  central  projection  kno^n  as  the  columella. 
This  may  be  mistaken  for  the  spore  case  itself. 

The  spores  are  oval  and  larger  than  those  of  Penicillium, 
consisting  of  a  sac  enclosing  protoplasm  and  a  nucleus.  When 
sown  in  a  proper  medium  they  send  out  hyphfe  and  produce  a 
new  plant.  The  spores  are  at  first  colorless,  but  when  ripe  are 
colored  and  give  the  black  appearance  to  the  sporangia. 

The  hyphae  are  cylindrical  threads,  longer  and  larger  in 
diameter  than  those  of  Penicillium,  and  when  young  have  no 
dividing  partitions,  so  that  each  hypha,  however  long,  with 
all  its  branches,  forms  a  single  cell.  In  old  growths,  partitions 
may  be  found  after  the  production  of  sporangia.  The  hyphae 
contain  granular  protoplasm  with  vacuoles  and  nuclei. 

Carefully  remove  some  hyphse,  white  (unripe)  and  dark 
spore  cases  with  a  teasing  needle  or  forceps  to  a  slide  and  keep 
them  spread  apart.  Add  a  drop  or  two  of  70  or  80  per  cent, 
alcohol  and  cover.  Water  causes  the  spore  cases  to  swell  and 
burst,  hence  should  not  be  used.  Examine  with  low  and  high 
power.  Note  the  hyphse,  spore  cases  (both  whole  and  broken), 
columella,  spores,  protoplasm,  etc.  Also  apply  fuchsin  and 
iodine  stains  and  note  the  effects.  Compare  with  Penicillium 
as  to  its  dimensions  of  parts. 

When  moist  bread  is  allowed  to  become  mouldy  Mucor  is 
apt  to  be  the  first  growth.     Later  on  this  mould  will  die  away 


Fig.  31.— Mucor  stolonlfer.  a,  hypha ;  b,  ripe  spore  case ;  c,  collar  or  remains  of 
broken  wall  of  spore  case ;  d,  columella  or  dome-like  partition  separating  spore 
case  from  the  cavity  of  its  stalk,  g ;  f,  broken  spore  case  with  spores  ;  e,  young 
spore  case,  spores  not  yet  formed  ;  h,  spores. 

and  Penicillium  or  Eurotium  will  get  the  upper  hand  and  flour- 
ish. The  source  of  the  mould  is  the  spores  which  float  about 
in  the  air  or  are  present  in  water. 


Claviceps  Purpurea  (Ergot  op  Rye). 

Claviceps  is  a  genus  of  the  fungi  whose  different  species  pro- 
duce Ergot  grains  on  various  kinds  of  grasses.  The  hyphae  of 
the  species  Claviceps  purpurea  begin  their  development  on  the 


Ergot  of  Rye. 


47 


surface  and  interior  of  the  ovary  of  the  flowers  of  Rye  as  deli- 
cate filaments.  After  a  certain  time  the  fungus  begins  to  form 
a  dense  mass  of  thick,  hard,  dark  purple  hyphge,  which  gradu- 
ally destroy  and  take  the  place  of  the  cells  of  the  ovary  until 
finally  there  is  scarcely  anything  left  of  the  latter.  This  hard 
mass,  known  as  the  sclerotium  stage,  constitutes  the  official 
Ergot  of  Rye.  This  is  a  resting  stage,  the  grain  lying  dormant 
until  spring,  when,  if  placed  in  warm,  damp  soil,  there  arises  a 
number  of  stalked  bodies  with  globular  heads  in  which  spores 
are  produced.  If  these  spores  be  carried  by  the  wind  to  the 
flowers  of  Rye  they  develop  and  produce  ncAV  grains  in  the  man- 
ner just  described. 

Wrap  a  few  large  grains  in  moistened  filter  paper  and  keep 
in  a  corked  bottle  for  several  hours.     By  this  time  they  will 


Q&oOa 


'.0.3; 


■^aOcOSqoQ^g 


Fig.  32.— Ergot  of  Rye,  Transverse  Section  (Vogl). 

have  changed  from  a  brittle  to  a  flexible  state.  Place  half  of  a 
grain  between  the  two  halves  of  a  piece  of  elder  pith,  clamp  in 
a  microtome  and  cut  very  thin  transverse  sections  through  the 
pith  and  place  them  in  water.  Mount  in  water  on  a  slide  and 
examine  with  low  and  high  power.  The  margin  of  the  section 
consists  of  smaller  cells  with  brown  contents.  Within  this 
border  the  cells  are  larger  and  lighter  in  color,  rounded  or  oval, 
having  thick  cell-walls  and  oily  contents.  In  chloral  hydrate 
solution  the  cells  become  clearer  and  the  oil  collects  in  larger 
globules.  In  longitudinal  section  the  appearance  is  nearly  the 
same  as  in  the  transverse. 


48  Vegetable  Histology. 

CHAPTER    YIII. 
The  Tissues  of  the  Higher  Plants. 

The  lessons  thus  far  have  been  given  to  the  study  of  some  of 
the  simple  plants,  for  the  purpose  of  giving  an  idea  of  the 
nature  of  the  lowest  forms  of  plant  life  as  well  as  familiarizing 
the  student  with  the  use  of  the  microscope  and  the  manipula- 
tion of  objects  on  the  slide.  Some  of  the  plants  studied  play  an 
important  role  in  the  life  economy,  for  example,  yeast,  bacteria, 
moulds,  and  thus  deserve  close  study.  While  studying  these 
plants  we  have  learned  what  is  meant  by  a  plant  cell,  and  the 
subsequent  lessons  will  be  devoted  to  a  study  of  the  various 
kinds  of  cells  and  webs  of  cells  known  as  "tissues,"  found  in 
the  most  highly  developed  and  complex  plants,  the  Phane- 
rogamia  or  flowering  plants. 

The  peculiarity  of  these  is  that  there  is  a  great  division  of 
labor,  with  corresponding  tissues  and  organs,  which  have  been 
differentiated  from  a  fundamental  tissue.  Thus  we  have  a  leaf, 
an  organ  for  manufacturing  protoplasm  and  starch ;  the  flower, 
which  is  the  reproductive  organ;  the  stem,  the  channel  for 
conveying  sap;  the  roots  for  imbibing  water  and  nourishment. 
The  cells  are  differentiated  into  distinct  tissues.  On  passing 
down  to  the  lower  series  of  plants  these  tissues  become  simpler 
until,  finally,  in  the  Thallophyta  and  most  of  the  Bryophyta  we 
have  no  distinction  of  tissues  at  all.  Those  plants  consist  of 
a  homogeneous  mass  of  cells,  as  we  have  seen  in  the  case  of 
algfe  and  moulds. 

The  various  tissues  or  cell-webs  of  the  flowering  plants, 
namely,  epidermal,  ground,  fibro-vascular,  stony,  etc.,  are  all 
derived  from  cells  that  were  at  one  time  all  alike.  By  various 
physical  and  chemical  modifications  the  cells  come  to  differ 
from  one  another  and  thus  to  give  rise  to  the  different  tissues. 
The  cells  of  stony  tissue,  as  found  in  shells  of  nuts,  were  once 
like  the  soft  cells  of  a  leaf,  but  they  became  subsequently  hard- 
ened and  modified. 

A  cell  has  been  defined  as  a  nucleated  mass  of  protoplasm. 
It  may  or  may  not  possess  a  cell-wall  of  different  composition. 
With  rare  exceptions  in  vegetable  cells  such  a  wall  is  present, 
while  most  animal  cells  are  destitute  of  it ;  but  in  all  essential 
respects  animal  and  vegetable  cells  resemble  each  other.  Cells 
are  the  structural  units  of  the  organism.  All  plant  bodies  are 
composed  of  cells  or  of  these  together  with  the  products  of  cell 
activity.  Within  the  compass  of  the  cell  occur  all  those  essen- 
tial phenomena  which  are  called  vital;  the  life  of  a  plant  resides 
in  its  cells;  the  sum  of  the  activities  it  exhibits  is  the  sum  of 
the  activities  of  its  component  cells. 


Typical  Vegetable  Cell.  49 

Vegetable  cells  are,  on  the  average,  not  more  than  one  five- 
hundredth  or  one  six-hundredth  of  an  inch  in  diameter,  though 
in  some  cases  they  are  large  enough  to  be  distinctly  seen  by  the 
unaided  eye,  as  in  the  flesh  of  the  Watermelon  and  the  pith  of 
Elder ;  in  rare  instances,  as  the  internodal  cells  of  Chara,  they 
may  even  be  more  than  an  inch  long.  Some  cells,  on  the  other 
hand,  are  so  small  as  to  be  barely  visible  under  the  highest 
powers  of  the  microscope,  for  example,  some  bacteria. 

The  primary  form  of  cells  appears  to  be  that  of  a  sphere  or 
spheroid,  but  commonly,  especially  in  the  tissues  of  the  higher 
plants,  they  acquire  forms  quite  different  from  this,  and  even 
within  the  limits  of  the  same  organism  the  shapes  may  be  ex- 
ceedingly various.  This  may  be  due  to  mutual  pressure,  to 
unequal  growth  caused  by  the  unequal  operation  of  various 
physical  forces,  as  gravitation,  light,  etc.,  or  to  other  influence. 
Cells,  like  the  organs  of  which  they  are  components,  undergo 
many  modifications  of  form  and  structure,  adapting  them  to 
different  uses.  The  cells  which  make  up  the  body  of  a  plant 
are  comparable  to  the  human  units  which  make  up  society.  A 
plant  is  a  community  or  republic  of  cells,  and,  to  understand  it, 
one  must  understand  the  individuals  that  compose  it. 

Typical  Vegetable  Cell. 

As  all  the  different  kinds  of  cells  that  go  to  make  up  the 
various  tissues  of  a  plant  are  derived  from  cells  that  are  at  one 
time  all  alike,  we  will  begin  by  a  consideration  of  these  primi- 
tive or  typical  cells,  and  afterwards  study  the  various  modi- 
fications. 

Peel  off  the  skin  or  epidermis  from  the  convex  surface  of  an 
onion  scale  by  making  a  cross  incision  and  catching  the  skin 
between  the  thumb  and  the  knife  or  razor  edge.  Be  careful  not 
to  draw  along  with  the  epidermis  any  of  the  thick  underlying 
flesh  of  the  scale.  Mount  a  piece  of  the  skin  about  a  quarter- 
inch  square  in  water  on  a  slide,  cover  carefully  with  a  glass  so 
as  not  to  include  any  air-bubbles. 

Examine  with  low  power.  Very  little  will  be  made  out. 
There  is  a  fine  and  somewhat  irregular  network.  This  is  due  to 
an  aggregation  in  a  single  layer  of  a  number  of  cells,  the  net- 
work of  lines  being  the  bounding  cell-walls,  which  are  so  nearly 
transparent  as  to  be  almost  invisible.  If  the  light  be  properly 
dimmed  there  may  be  seen  in  each  cell  a  small  denser-looking 
body  which  is  called  the  nucleus,  and  perhaps  faintly  granular 
matter.  The  cells  are  filled  with  a  semi-liquid  matter,  which, 
however,  is  too  transparent  to  be  seen. 

Examine  the  various  parts  of  a  cell  with  high  power.  The 
details  are  somewhat  diflflcult  to  make  out  because  of  the  trans- 
parency of  the  cell  contents.     This  is  very  often  the  case  with 


50 


Vegetable  Histology. 


living  cells,  but  the  difficulty  can  be  overcome  by  killing  the 
protoplasm  or  staining  it. 

Raise  the  cover  glass,  add  a  drop  of  iodine  stain  and  allow  a 
few  moments  for  it  to  penetrate  the  cells,  then  cover  and  exam- 
ine again. 

The  cell-walls,  scarcely  stained,  are  distinctly  visible.  In 
mature  cells  aggregated  to  form  tissues,  the  common  cell-wall 
between  two  cells  is  made  up  of  two  like  portions  separated  by 
a  layer  of  a  slightly  different  chemical  substance,  which  is 
more  soluble  in  reagents  than  the  rest  of  the  wall  and  shows 
different  reactions  with  test  reagents  and  is  known  as  the 
middle  lamella.  Next  to  the  cell-wall  is  a  layer  of  protoplasm, 
granular  and  deeply  stained  yellowish  brown,  called  the  prim- 
ordial utricle.  Somewhere  within  the  cell  will  be  seen  a  dense 
body,  the  nucleus,  surrounded  by  protoplasm  and  connected  by 
strings  of  protoplasm  with  the  utricle.     Between  the  strings 


A    jQ^'':k 


Fig.  33.— Cells  of  Onion  epidermis.  A,  surface  view,  a,  nucleus ;  c,  cell-wall ;  d, 
vacuole ;  b,  space  where  protoplasm  has  shrunken  from  cell-wall ;  e,  primordial 
ntricle.    B,  cross-section,    a,  cuticle ;  b,  cellulose  portion  of  cell-wall. 

are  vacuoles,  clear  spaces  filled  with  cell-sap.  The  nucleus 
contains  several  smaller  bodies,  which  are  little  nuclei  or 
nucleoli  (plural  of  nucleolus).  In  some  cells  the  protoplasm 
may  have  shrunken  away  from  the  cell-wall  at  one  end,  leaving 
a  clear,  apparently  empty  space.  The  cells  dovetail  into  one 
another,  leaving  no  intercellular  spaces,  but  forming  a  close 
impervious  layer. 

Like  epidermal  cells  in  general,  the  cells  just  described  are 
rather  flat,  but  that  is  not  apparent  in  surface  view.  This  fact 
is  brought  out  in  sections  cut  vertical  to  the  surface  of  the 
epidermis,  which  is  best  done  by  cutting  through  several  scales 
of  the  onion.  The  appearance  is  given  in  Fig.  33.  The  cells  are 
oblong  in  shape  and  the  outer  wall  is  somewhat  thickened. 

Raise  the  cover  glass  from  the  section  that  was  stained  with 
iodine,  remove  the  excess  of  fluid  and  add  a  drop  of  sulphuric 
acid  (2  vols.  cone,  acid  to  1  vol.  water),  after  a  moment  replace 
the  cover  glass  and  examine.     The  cell-walls  are  stained  a  deep 


Typical  Vegetable  Cell.  51 

blue,  proving  that  the}'  are  cellulose  in  nature.  A  light  line, 
not  blue,  but  of  a  yellow  color,  is  in  the  middle  of  the  cell-walls 
and  marks  off  the  boundary  of  each  cell.  This  line  is  the  middle 
lamella.  It  is  composed  chiefly  of  calcium  pectate.  The  cellu- 
lose portions,  as  well  as  the  middle  lamella,  gradually  swell, 
finally  dissolve  and  disappear. 

If  a  cross-section  of  the  epidermis  be  treated  as  above  de- 
scribed the  thickened  outer  wall  will  be  seen  to  consist  of  two 
layers,  an  inner  one,  which  is  cellulose  and  stains  blue,  and  an 
outer  one,  which  stains  yellowish  or  brownish.  This  outer 
layer  is  called  the  cuticle  and  is  composed  of  a  substance  known 
as  cutin  (cork  substance),  which  does  not  dissolve  in  the  acid. 
Cutin  is  very  resistant  to  reagents  and  thus  forms  an  excellent 
protecting  layer.    The  cuticle  is  represented  in  Fig.  33. 

The  typical  cell  just  described  is  somewhat  advanced  from 
the  earliest  stage  of  a  cell,  known  as  the  primary  meristem  cell. 
In  such  very  young  cells  the  wall  is  ex- 
ceedingly   thin    and    apparently    homo- 
geneous, the  vacuoles  are  absent  and  the 
entire   area  enclosed   by   the   wall   ap- 
pears to  be  filled  with  protoplasm  (Fig. 
34).    As  the  cell  grows  older  its  wall 
becomes  thicker  and   differentiated,   as    pig734~i;very  young  ceiis. 
described  above.    By  the  expansion  of  the 

wall  the  cavity  of  the  cell  increases  faster  than  the  contained 
protoplasm,  the  latter  imbibes  more  water  than  it  is  capable 
of  holding  in  solution,  and  thus  sap  cavities  or  vacuoles  are 
formed,  which,  at  the  maturity  of  the  cell,  often  occupy  more 
space  than  the  protoplasm  itself.  Finally,  when  the  cell  is 
quite  old  its  living  contents  disappear'  altogether  and  the  cell 
is  dead  matter. 

As  the  cells  develop  to  form  the  various  tissues,  a  number  of 
substances  make  their  appearance  in  the  contents.  Some  of 
these  are  chlorophyll  bodies,  aleurone  grains,  starch,  fatty  oils 
and  fats,  calcium  salts,  glucosides,  alkaloids,  sugar,  bitter  prin- 
ciples, tannin,  resins,  gums,  inulin,  etc.  Some  of  these  will  be 
studied  in  later  lessons. 


52  Vegetable  Histology. 

CHAPTER    IX. 

Tissues  of  Variously  Modified  Cells  as  Found  in  Higher 

Plants. 

While  it  is  true  that  all  the  essential  phenomena  which  we 
call  vital  are  manifested  within  the  compass  of  a  single  cell,  it 
is  also  true  that  the  manifestation  is  feeble  in  comparison  with 
that  exhibited  by  cell  aggregates,  where  there  is  division  of 
labor  among  cells.  All  the  higher  plants  are  such  aggregates 
of  cells.  They  are  made  up  of  millions  of  them,  and  their  life 
is  not  the  mere  aggregate  life  of  cells  precisely  alike,  but  rather 
that  of  sets  of  cells  that  have  come  to  differ  from  each  other  in 
form  and  function,  but  all  subserving  the  life  of  the  whole 
organism. 

These  cell  groups,  which  differ  from  each  other  in  ways  more 
or  less  important,  but  each  of  which  is  composed  of  similar 
cells,  are  called  tissues.  There  is  a  great  variety  of  tissues,  the 
individual  cells  of  which  differ  more  or  less  markedly  from  the 
typical  cell  already  described. 

It  must  be  remembered,  however,  that  all  these  tissues  orig- 
inate from  a  single  cell,  and  that  each  cell  of  the  mature  plant, 
however  great  its  deviation  from  the  typical  form,  approximates 
the  latter  very  closely  in  its  early  stages  of  development. 

The  various  kinds  of  tissues  are  classified  into  four  groups 
or  series. 

1.  Parenchyma;  ordinary  soft  ground 
tissue. 

2.  Collenchyma;  thick-angled  tissue. 

3.  Sclerotic  parenchyma;  stony  tissue. 

4.  Epidermal  tissue. 

5.  Endodermal  tissue. 

6.  Suberous  or  corky  tissue. 

1.  Wood  or  libriform  tissue. 

2.  Tracheids  or  vasiform  cells. 

3.  Ducts  or  vascular  tissue. 
J:.  Hard  bast  or  bast-fibres. 

III.  Sieve  Series,  including  only  sieve  or  cribriform  tissue. 

IV.  Laticiferous  Series,  including  laticiferous  or  milk- 
tissue,  which  may  be  simple  or  complex. 

Not  all  of  the  tissues  are  alive  at  maturity.  Some  are  dead 
and  merely  serve  as  supporting  tissues.  All  of  the  prosen- 
chymatous  series  are  of  this  nature,  being  devoid  of  protoplasm 
and  mechanical  in  function.  Likewise  sclerotic  and  suberous 
tissues.  Any  one  of  the  higher  plants  will  contain  most  of  the 
above  tissues.  If  we  examine  steins  of  plants  we  find  that  the 
tissues  are  not  arranged  indiscriminately,  but  always  follow  a 
definite  order.  This  order  in  Phanerogams  is  different  from 
that  in  Cryptogams,  in  exogenous  flowering  plants  it  is  dif- 
ferent from  that  in  endogenous  ones.     But  in  any  particular 


I.  Parenchymatous 
Series. 


II.    Prosenchymatous 
Series. 


Tissues  in  Higher  Plants.  53 

case  the  order  is  always  the  same.  Exogens  are  dicotyledonous 
plants  and  are  characterized  by  having  their  stems  sharply 
divided  into  bark  and  central  wood  cylinder,  while  endogens, 
or  monocotyledonous  plants,  have  no  sharply -defined  bark  and 
wood  core. 

To  get  an  idea  of  the  various  kinds  of  tissues  and  the  par- 
ticular order  of  arrangement  always  met  with  in  exogens,  the 
stem  of  the  common  Geranium  serves  very  well.  The  stem  of 
most  any  other  exogen  would  answer,  as  the  Bittersweet,  Elder, 
Willow,  Sycamore,  Maple,  Yellow-Parilla,  etc. 

Make  several  thin  cross-sections  of  a  common  Geranium  stem, 
free-hand  or  by  the  microtome ;  place  one  on  a  slide,  add  a  few 
drops  of  chlor-zinc-iodine  solution  and  cover.  Examine  with 
low  power. 

The  first  thing  that  strikes  one  is  that  the  cells  differ  in  size 
and  shape,  in  compactness  of  arrangement,  in  thickness  of  cell- 
walls,  in  contents,  besides  that  some  stain  blue  while  others 
stain  brown. 

Going  from  the  exterior  towards  the  center  the  very  first 
layer  of  cells  is  the 

1.  Epidermis  or  external  bounding  tissue,  a  single  tier  of 
closely  laid  and  similar  cells,  interspersed  with  hairs  and  having 
their  outer  walls  thickened  into  a  cuticle.  If  the  stem  be  too 
old,  the  epidermis  may  not  be  present. 

2.  Beneath  the  epidermis  are  several  tiers  of  tabular,  brick- 
shaped  cells  in  radial  rows,  the  cells  of  the  outer  layers  are 
empty  and  their  walls  stain  brown,  while  the  inner  cells  may 
contain  protoplasm  and  the  walls  show  some  blue  color.  This 
is  the  cork  layer. 

3.  Collenchpma,  next  to  the  cork,  consisting  of  cells  quite 
different  in  shape  and  arrangement  from  the  cork  cells,  with 
cellulose  walls,  as  shown  by  the  blue  color  with  chlor-zinc- 
iodine,  rounded  or  polygonal  and  thickened  at  the  angles  where 
the  cells  join.  The  cells  are  rich  in  protoplasm.  They  are  not 
as  well  developed  nor  as  conspicuous  in  the  Geranium  as  in 
some  other  plants. 

4.  The  next  layer  of  cells  is  ordinary  parenchyma  or  ground 
tissue^  a  broad  zone  of  large  cells,  with  walls  very  thin  and 
uniform  and  stained  blue  (cellulose),  rich  in  protoplasm  and 
starch  contents,  globular  in  shape,  with  small  angular  inter- 
spaces at  the  angles  where  the  cells  meet. 

5.  Bast  fibres^  next  to  the  parenchyma,  a  zone  of  much 
smaller,  angular,  very  thick-walled  cells,  stained  a  deep  brown 
(lignified  walls),  compactly  arranged  and  free  from  proto- 
plasmic and  starchy  contents.  The  bast  fibres  are  dead  and  act 
only  as  mechanical  tissue.  The  cell-walls  are  stained  brown 
like  those  of  the  cork  cells,  which  might  indicate  that  they  are 
composed  of  the  same  material.  That  this  is  not  so  may  be 
shown  by  adding  to  a  fresh  section  a  drop  of  phloroglucin 


u 


Vegetable  Histology. 


reagent,  followed  after  a  time  by  strong  hydrochloric  acid.  The 
cork  cells  remain  unchanged,  while  the  bast  cells  assume  a  fine 
reddish-purple  color.  The  chemical  substance  composing  the 
bast  cells  is  known  as  lignin. 

The  fibrous  nature  of  the  cells  is  seen  only  in  longitudinal 
section. 

6.  The  next  layer  is  a  not  very  broad  zone  of  small-celled 
tissue  with  blue  stained  walls.  Under  high  power  this  is  com- 
posed of  two  sub-layers;  the  outer  one  consists  of  larger  cells, 
more  rounded,  of  unequal  size,  irregularly  arranged  and  made 
up  of  two  kinds  of  thin-walled  cells,  sieve  tissue  and  a  variety 
of  parenchyma.     These  constitute  the  so-called  soft  hast. 

7.  The  inner  layer  of  the  two  sub-layers  is  composed  of  very 
small  cells,  rich  in  protoplasm,  very  closely  packed,  in  radial 
rows.  These  are  primary  meristem 
cells  and  form  the  so-called  cam- 
bium zone  of  the  stem.  It  is  in  this 
zone  where  growth  takes  place  most 
actively,  by  which  the  stem  con- 
tinues to  increase  in  diameter. 
When  the  bark  is  stripped  from  a 
stem  the  rupture  takes  place  in  this 
succulent  fragile  zone. 

8.  Next  the  cambium  is  a  layer 
more  or  less  broad,  according  to  the 
age  of  the  stem,  composed  of  cells 
for  the  most  part  like  the  bast  fibres 
(see  5)  and  stained  brown.  These 
are  wood  or  libriform  fibres  (ligni- 
nified) .  Scattered  among  them  are 
other  cells  of  larger  diameter  whose 
walls  are  also  thickened  and  stain- 
ed brow^n.  These  constitute  the  vasi- 
form  or  vascular  tissue,  composed 
of  ducts  and  tracheids  of  vari- 
ous kinds.  Their  function  is  to 
strengthen  and  also  to  convey  nu- 
triment. All  these  cell-walls  are 
lignified  and  behave  like  the  bast 
fibres  towards  phloroglucin  stain. 

9.  Interior  to  the  wood  circle  is 
the  pith,  composed  of  large-celled 
parenchyma,  with  large  inter-cellu- 
lar  spaces   and   starchy   contents. 

The     cells      on      the      exterior     are      ^^e-  35.— Geranium  stem,  cross-sec- 

n  i-t  ii  1  T        ji  tion.     a,    cork    cells:    b,    collen- 

Smaller     than     those     towards     the         chyma ;  c,  parenchyma  of  middle 

center  and  more  compactly  arrang-       J^^'^i^bi^*^^  foZV  %^°rfng*'o^ 

ed.      Many  of  the  cells  of  the  pith         mixed  wood  fibres'  and  vessels ; 

and   the   outer   parenchyma    layer       Jhyma  c?n "  of*  pfth^TnJstffr"' 


Parenchyma  Tissue.  55 

contain  dark  masses,  which,  under  the  high  power,  are  seen  to 
be  stellate  crystals. 

Make  a  longitudinal  radial  section  and  examine  as  before. 
The  cork  cells  look  about  the  same  as  in  cross-section;  the 
collenchyma  cells  are  elongated ;  the  bast  fibres,  wood  cells  and 
ducts  are  very  much  elongated  and  oblique-ended  or  tapering. 

Examine  cross-sections  of  other  exogenous  plants. 

Make  permanent  mounts  of  sections  stained  in  haematoxylin 
and  fuchsin,  also  in  methyl-green  and  eosin. 


CHAPTER    X. 

Parenchyma  Tissue. 

In  the  parenchymatous  series  of  tissues  the  cells  are  less 
modified,  at  least  in  shape,  from  very  young  cells  than  those  of 
other  tissues.  They  mostly  retain  to  maturity  the  character- 
istics of  the  living  cell,  namely,  protoplasm  and  nucleus  and  the 
power  to  form  new  cells  by  division.  In  some  cases  they  be- 
come elongated  and  somewhat  fibrous,  but  more  commonly  they 
are  not  much  longer  than  broad,  and  have  their  ends  square  or 
rounded  rather  than  oblique  or  tapering.  In  most  of  the  tis- 
sues of  the  parenchymatous  series  the  cells  have  thin  walls,  but 
in  some  instances  the  cell-walls  are  thickened  by  cellulose, 
cutinous  or  ligneous  deposits. 

Ordinary  parenchyma  or  soft  ground  tissue  is  the  least  modi- 
fied and  most  abundant  of  all  the  plant  tissues.  The  walls  are 
thin  and  frequently,  though  not  always,  composed  of  unmodified 
cellulose,  commonly  spheroidal  or  polyhedral  in  form  and  the 
longitudinal  rarely  exceeds  the  transverse  diameter.  It  in- 
cludes most  of  the  soft  tissues  of  plants,  such  as  the  green  cells 
of  the  leaf,  the  cells  of  pith,  a  considerable  portion  of  the  cells 
of  bark,  etc.  Sometimes  the  cell-walls  are  unequally  thickened, 
so  as  to  present  the  appearance  of  markings  of  various  kinds; 
indeed,  they  are  seldom  of  uniform  thickness,  but  commonly 
their  membranous  character  and  transparency  make  them 
appear  so. 

For  the  study  of  ordinary  parenchyma  cells  the  young 
Geranium  stem,  studied  in  Chapter  IX,  would  serve  very  well, 
but  for  variety  sections  from  the  soft  green  parts  of  other  plants 
may  be  taken,  such  as  the  petiole  of  the  Begonia,  the  Pumpkin 
stem,  etc. 

Let  cross-sections  be  examined  first  in  water,  then  stained 
with  iodine  and  chlor-zinc-iodine. 

The  description  of  the  parts  of  a  typical  cell  given  in  Chapter 
VIII  answers  very  well  for  parenchyma  cells.  A  structure  not 
noted  there  is  present  in  all  green  parenchyma  cells,  namely, 
small  rounded  granules  stained  deep  brown,  the  chloroplasts  or 


56  Vegetable  Histology. 

chlorophyll  bodies.  In  the  fresh  unstained  cell  these  are  green. 
The  chlorophyll  granules  give  the  green  color  to  leaves,  etc. 

Another  difference  is  the  presence  of  small  intercellular 
spaces  between  the  cells,  not  found  in  the  onion  epidermis  or  any 
epidermis  in  fact.  Starch  grains  also  are  often  present,  which 
assume  a  deep  blue  color  with  iodine,  appearing  almost  black. 
Under  high  power  the  walls  that  have  been  stained  blue  by 
chlor-zinc-iodine  appear  finely  punctate  with  nearly  colorless 
dots.  These  are  thin  places  or  pits  in  the  walls.  Unlike  the 
epidermal  cells  of  the  onion,  parenchyma  cells  are  nearly  always 
globular,  as  may  be  seen  from  the  cross  and  longitudinal  sec- 
tions of  the  Geranium  stem  (Fig.  35.) 

Parenchyma  cells  are  not  always  globular  and  closely  packed, 
but  occur  at  times  in  modifications,  both  as  to  shape  as  well  as 
arrangement.     These  modified  forms  are — 

Stellate,  or  star-shaped  cells. 

Folded — the  cell-walls  have  internal  folds. 

Spongy — the  cells  are  very  loosely  arranged. 

Palisade — the  cells  are  elongated  and  arranged  like  posts, 
seen  in  leaves. 

Pitted — the  cell-walls  marked  by  thin  places  of  various  dimen- 
sions and  shapes. 

Folded  cells  will  be  seen  later  in  pine  needles,  spongy  and 
palisade  cells  in  leaves.  For  pitted  cells,  several  plants  might 
be  chosen,  but  a  convenient  one  is  the  plant  known  as  the  Sago 
Palm  (Cycas  revoluta). 

Examine  a  cross-section  of  the  petiole  unstained  in  water. 
Under  high  power  thick-walled  rounded  parenchyma  cells  will 
be  found  with  a  number  of  transparent,  rounded  areas  looking 


Fig.  36.— Pitted  parencliyma  celis. 

like  holes  in  the  wall.  These  are  thin  portions  of  the  otherwise 
thickened  walls  and  not  holes.  On  the  edges  the  pits  cause  a 
beaded  appearance  (Fig.  36).  Phloroglucin  shows  that  the 
walls  are  somewhat  lignified,  giving  a  red  color. 

Beautiful  permanent  mounts  may  be  made  by  staining  in 
aqueous  fuchsin  solution,  passing  the  section  through  70  per 
cent.,  90  per  cent,  and  absolute  alcohol,  then  into  eosin  oil  of 
cloves  for  ten  or  fifteen  minutes  and  finally  mounting  in  balsam. 
The  thin  areas  of  the  wall  will  be  stained  pink  by  eosin  and  the 
thicker  portions  red  by  fuchsin. 


COLLENCHYMA    CeLLS.  57 

CHAPTER    XI. 

COLLENCHYMA   CeLLS. 

Collenchyma,  or  thick-angled  tissue,  is  closely  related  to 
ordinary  parenchyma,  but  the  cells  are  more  elongated,  often 
five  or  six  times  longer  than  broad,  prismatic  in  shape  and  thick- 
ened at  the  angles.  The  thickenings  are  usually  not  lignified 
and  the  cells  contain  protoplasm,  a  nucleus  and  more  or  less 
chlorophyll.  They  are  never  found  elsewhere  than  in  close 
proximity  to  the  epidermis  or  rarely  in  a  similar  relation  to 
^ndodermal  tissue,  and  one  of  the  uses  of  the  cells  is  evidently 
that  of  giving  strength  to  the  epidermis.  Sometimes  collen- 
chyma  forms  a  continuous  circle,  as  in  the  petiole  of  Begonia 
and  Grape;  at  other  times  it  forms  longitudinal  bands,  as  in 
stem  of  Yellow  Dock  and  Cow-parsnip. 

The  thickenings  of  the  cell-walls  in  some  plants  are  excessive 
and  strongly  diminish  the  cell  lumen ;  in  others  they  are  slight. 
Sometimes  they  are  confined  to  the  angles,  while  at  others  they 
extend  to  a  less  degree  to  the  rest  of  the  cell-wall. 

The  petioles  of  Burdock,  Begonia  or  any  species  of  Grape 
afford  good  examples  for  the  study  of  collenchyma. 

The  petiole  of  Burdock  is  bluntly  angular,  and  seven  ribs, 
darker  in  appearance  than  the  rest  of  the  surface,  may  be  seen 
running  lengthwise  and  forming  the  angles.  These  are  the  ribs 
of  collenchyma  cells.  Make  cross-sections  and  examine  first  in 
water  with  low  and  high  power.  The  cells  occur  in  patches 
just  beneath  the  epidermis  at  the  angles  of  the  section.  The 
cell-walls  are  much  thickened,  especially  at  the  corners,  and  are 
strongly  glistening  (refractive),  in  this  respect  showing  a 
marked  contrast  to  the  neighboring  larger  parenchyma  cells. 
The  cavities  of  the  cells  look  dark  by  contrast  with  the  shining 
cell-walls.  At  the  border  between  collenchyma  and  paren- 
chyma tissue  the  first  line  of  cells  of  the  latter  tissue  have  one- 
half  of  their  walls  thickened,  while  the  other  half  is  thin. 

The  walls  of  the  collenchyma  cells  are  marked  with  delicate 
stratification  lines.  Such  lines  are  common  to  thick  walls 
generally  and  are  due  to  the  different  layers  containing  differ- 
ent amounts  of  water,  as  may  be  proved  by  immersing  in  strong 
alcohol,  which  removes  all  the  water,  when  the  lines  will  dis- 
appear. Collenchyma  is  absent  in  most  monocotyledonous 
plants.  In  longitudinal  section  the  cells  present  quite  a  differ- 
ent appearance.  The  thickenings  appear  as  long  narrow  bands, 
the  cells  are  elongated,  some  being  twice,  others  five  to  six 
times  as  long  as  broad,  and  blunt-ended.  In  some  plants  coflen- 
chyma  tends  strongly  to  fibrous  tissue,  being  very  long  and 
greatly  thickened  and  taper-pointed. 


58 


Vegetable  Histology. 


A  good  way  to  get  a  longitudinal  view  of  the  cells  is  to  strip 
off  the  epidermis  from  a  Lily  petiole ;  the  collenchyma  cells  will 
come  off  with  it  and  may  readily  be  studied  in  water  or  iodine 
solution. 

In  iodine  solution  the  collenchyma  cells  are  seen  to  contain 
protoplasm  and  a  nucleus.  If  the  excess  of  iodine  solution  be 
removed  and  sulphuric  acid  reagent  be  added,  the  cell-walls  will 
swell  and  assume  a  blue  color,  which  shows  that  they  are  cellu- 
lose in  nature.     Chlor-zinc-iodine  solution  gives  the  same  result. 

While  studying  collenchyma  in  the  petiole  of  Burdock  atten- 
tion should  be  given  to  the  whole  section,  as  another  illustration 
of  the  dicotyledonous  type  of  stem  structure.  A  number  of  dis- 
tinct moccasin-like  areas  may  be  observed,  arranged  in  a  single 
circle  and  separated  from  each  other  by  plates  of  ordinary 
parenchyma  cells.  These  are  the  fihro-vascular  hundles,  con- 
sisting of  bast  and  wood  fibres  and  tracheary  vessels,  just  as  in 


Fig,  37.— Sketch  of  cross-section 
of  petiole  of  Burdoclt.  a, 
patch  of  collenchyma  ;  b,  ring 
of  bundles ;  c,  pith  with  hole 
In  center. 


Fig.  38.  —  Cross-section  of  collen- 
chyma of  Burdock  magnified, 
showing  thickened  angles  of  cell- 
walls   (Bastin). 


the  Geranium  stem.  But  as  in  the  case  of  succulent  plants  in 
general,  so  here  the  vascular  bundles  have  not  coalesced,  so  that 
there  is  no  continuous  ring  of  bast  fibres,  cambium  zone  and 
wood  fibres  and  vessels,  as  is  the  case  in  the  Geranium.  Next 
to  the  circle  of  bundles  comes  the  pith,  the  center  of  which  is 
occupied  by  a  cavity.     Cork  cells  are  absent. 

Let  a  cross-section  of  Burdock  soak  a  few  minutes  in  phloro- 
glucin  solution,  then  add  hydrochloric  acid  and  note  that  the 
only  lignified  material  occurs  in  the  vascular  bundles  in  the 
fibres  and  vessels. 


Sclerotic  Cells.  59 

CHAPTER    XII. 

Sclerotic  Cells. 

The  cells  of  this  tissue  are  commonly  called  stone  or  grit  cells. 
The  cells  differ  from  ordinary  parenchyma  ones  in  having  the 
walls  excessively  thickened,  so  much  so  frequently  that  the 
cavity  of  the  cell  is  nearly  obliterated.  Every  gradation,  how- 
ever, msij  be  observed  between  these  and  ordinary  parenchyma. 
The  walls  of  sclerotic  cells  are  usually  lignified  and  the  thicken- 
ing is  deposited  in  layers,  giving  the  appearance  of  concentric 
rings.  These  are  the  cells  which  give  the  great  hardness  to  the 
outer  coats  of  seeds  and  the  shells  of  nuts.  They  constitute  the 
gritty  particles  that  occur  in  the  flesh  of  some  fruits,  as  the 
pear  and  apple,  and  are  present  in  many  barks,  for  example, 
Cinnamon,  Oak,  Viburnum,  Cascara,  etc.  Stone  cells  are 
classed  by  some  with  parenchymatous  tissues  because  of  their 
origin  and  shape,  but  otherwise  they  have  very  little  in  com- 
mon with  parenchyma. 

Sclerotic  Tissue  op  Walnut  Shell. 

Use  a  piece  of  shell  that  has  been  softened  by  long  soaking  in 
10  per  cent,  alkali  and  washed  in  acidified  water.  Cut  thin 
sections  in  various  directions,  taking  care  not  to  let  the  razor 
run  too  deep.  Mount  in  water  and  examine  with  low  power, 
picking  out  the  thinnest  edge. 

There  will  be  seen  a  mass  of  rounded,  somewhat  polyhedral 
cells,  pressed  so  closely  together  that  no  intercellular  spaces 
are  visible.  The  walls  are  extremely  thickened  and  contain 
minute  dots,  as  well  as  radial  lines  connecting  the  small  cavity 
or  lumen  of  the  cell  with  the  middle  lamella.  In  this  section 
the  cells  are  almost  colorless. 

Put  on  high  power.  The  radial  lines  can  now  be  seen  to  be 
tubes,  and  the  dots  are  tubes  cut  cross-wise,  the  ends  appearing 
as  dots.  The  tubes  are  known  as  pore-canals  and  are  analogous 
to  the  pits  in  ordinary  parenchyma  cells.  They  probably  serve 
to  help  the  circulation  of  nutritive  fluids  from  one  cell  to 
another,  as  is  evidenced  by  the  fact  that  the  tubes  of  neighbor- 
ing cells  end  opposite  each  other. 

With  careful  examination  the  walls  are  seen  to  be  made  up 
of  concentric  layers.  These  are  made  more  distinct  by  adding 
a  drop  of  chloral-hydrate  solution  (5  chloral-hydrate  to  2  water) 
and  watching  closely  its  swelling  action.  The  lines  come  out 
plainly  at  first,  but  after  a  time  disappear,  owing  to  continued 
swelling. 

Examining  sections  cut  in  various  directions  from  the  shell, 


60  Vegetable  Histology. 

it  will  be  found  that  the  cells  have  the  same  general  appear- 
ance, showing  that  they  are,  in  form,  essentially  like  paren- 
chyma cells,  the  difference  being  that  the  walls  are  immensely 
thickened  in  stone  cells,  and  they  no  longer  take  part  in  the 
vital  processes  of  the  plant,  but  act  as  mechanical  tissue;  in 
fact,  in  some  cases  they  are  elongated  and  fusiform,  so  that  it  is 
difficult  to  distinguish  them  from  prosenchyma  fibres.  All 
gradations  are  found  between  parenchyma  and  stone  cells  and 
between  stone  cells  and  bast  fibres. 

Add  phloroglucin  and  hydrochloric  acid  to  a  section.  The 
walls  are  stained  red,  proving  lignified  material. 

Stone  cells  may  be  studied  to  better  advantage  by  isolating 
them  by  Schulze's  maceration  solution  as  directed  under  this 
reagent.  Stain  a  section  treated  thus  with  methyl-green  and 
mount  in  water,  and  tap  the  cover  glass  with  a  teasing  needle, 
when  the  cells  will  separate  in  virtue  of  the  middle  lamellas, 
which  unite  the  cells,  being  dissolved  away  by  the  maceration 
fluid. 

Examine  sections  cut  in  various  directions  from  cocoanut 
shell.  These  cells  have  a  natural  brownish-yellow  color,  and 
their  boundaries  are  more  easily  seen  than  in 
the  case  of  the  walnut  cells.  While  many  cells 
are  almost  circular  in  outline,  others  are  some- 
what elongated.  This  fact  indicates  that  the 
cells  tend  towards  the  fibrous  condition  and  run 
in  different  directions,  so  that  in  the  same  sec- 
tion some  are  seen  in  cross-section  and  appear  ^of  wal^u\°^hen.^* 
round,  while  others  are  seen  lengthwise  and 
appear  elongated.  The  cavities  are  filled  with  a  dark  material. 
In  general  structure  the  cells  resemble  those  of  the  walnut 
shell.  Ground  cocoanut  shells  are  used  frequently  as  an  adul- 
terant of  ground  spices  and  should,  therefore,  command  close 
study  by  the  student. 

To  study  the  stone  cells  as  they  occur  in  barks,  wrap  pieces 
of  the  bark  of  cassia  cinnamon  and  cascara  in  wet  filter  paper 
and  keep  in  a  closed  bottle  until  they  are  soft  enough  for  sec- 
tioning. Also  immerse  a  piece  of  viburnum  (stem)  bark  in 
dilute  (1  or  2  per  cent.)  alkali  until  soft,  and  finally  wash 
thoroughly  in  water.  Cut  cross-sections  between  pith  or  cork 
and  mount  in  water  or  chloral  solution.  Cinnamon  has  an 
interrupted  chain  of  colorless  stone  cells  running  tangentially 
about  midway  of  the  section.  Cascara  presents  large  masses 
of  yellow  stone  cells  at  about  one-third  the  width  of  the  section 
from  the  outer  edge.     Viburnum  has  similar  masses  of  stone 


Epidermal  Tissue. 


61 


cells  distributed  over  the  whole  section.  The  stone  cells  can 
easily  be  recognized  by  their  structure  as  well  as  by  the  red 
stain  they  assume  with  phloroglucin  and  hydrochloric  acid. 


Fig.  40.— Cassia  bark,  cross-section.  K,  stony  cork  cells ;  pr,  cortical  parenchyma, 
with  stony  parenchyma  cells ;  st,  stone  cells  forming  an  Interrupted  ring ;  b, 
bast  fibre ;  pb,  a  group  of  primary  bast  fibres ;  sch,  secretion  cell ;  m,  medullary 
rays;  s,  sieve  tubes  (Moeller). 


CHAPTER    XIII. 


Epidermal  Tissue. 

This  tissue  has  already  been  met  with  in  the  exercise  on  the 
onion  epidermis,  but  the  latter  is  not  quite  a  typical  example, 
as  there  are  no  stomata  or  breathing  pores  present,  which  are 
always  found  when  an  epidermis  is  exposed  to  the  air.  Since 
the  onion  scale  epidermis  is  not  exposed  to  the  air  there  is  no 
need  for  the  pores  and  they  are  consequently  absent.  In  other 
respects  the  epidermis  of  other  plants  resembles  very  much  that 
of  the  onion. 

The  tissue  constitutes  the  primary  covering  of  the  plant.     It 


62  Vegetable  Histology. 

usually  consists  of  one,  but  sometimes  of  two  or  three  layers  of 
cells.  The  cells  are  closely  packed  together,  leaving  no  inter- 
cellular space  except  the  breathing  pores,  and  commonly  they 
have  that  portion  of  the  cell-wall  which  faces  exteriorly  consid- 
erably thickened  and  cutinized  and  are  usually  flattened.  When 
seen  in  surface  view  they  often  appear  sinuous  or  irregular  in 
outline,  but  sometimes  they  are  straight-sided  and  regular. 
In  many  plants  they  are  somewhat  elongated  in  the  direction 
of  the  length  of  the  organ,  especially  in  the  cells  on  the  veins 
on  the  under  surface  of  leaves. 

The  cells  are  rich  in  protoplasm.  The  different  parts  that 
were  noted  in  the  onion  epidermis  are  noticed  just  as  plainly 
in  other  cases.  In  most  plants  there  are  no  chlorophyll  bodies 
present  in  the  epidermal  cells,  to  which  fact  is  due  their  trans- 
parency.   Ferns  are  exceptions  to  this. 

Stomata  or  Breathing  Pores. 

These  pores  are  minute  apertures,  usually  surrounded  by  a 
pair  of  crescent-shaped  cells  called  guard  cells.  These  are 
much  smaller  than  the  epidermal  cells  and  are  much  richer  in 
proteid  matters,  containing  a  nucleus,  protoplasm,  numerous 
chlorophyll  bodies  and  occasionally  oil  globules. 

By  means  of  the  pores  the  plant  exhales  the  superfluous  water 
taken  in  by  the  roots  and  the  excess  of  oxygen  not  used  by  it 
and  takes  in  the  carbon  dioxide  necessary  for  the  plant's  life. 
They  always  open  into  a  large  intercellular  space.  Thus  the 
outside  air  is  in  free  communication  with  the  whole  interior 
of  the  plant  stem  and  leaves,  since  the  air  circulates  freely 
through  the  intercellular  spaces  which  are  in  communication. 
Communication  with  the  interior  of  the  plant  takes  place  only 
through  the  pores,  since  the  cutinized  exterior  of  the  surface 
of  the  epidermal  cells  is  highly  impregnable  to  water  and  air. 
Hence  epidermis  is  an  excellent  protection  against  evaporation 
of  moisture  from  the  interior  of  the  plant. 

The  size  of  the  breathing  pore  is  regulated  by  the  guard  cells, 
which  expand  or  contract  according  as  they  absorb  or  give  off 
moisture.  The  thin  radial  walls  and  the  thickened  outer  and 
inner  walls  are  so  devised  by  nature  that  when,  in  hot,  dry 
weather,  moisture  is  given  off  by  the  guard  cells  to  the  air,  the 
concave  sides  enclosing  the  opening  straighten  out  and  thus 
close  it,  thereby  stopping  the  further  evaporation.  Similarly, 
when  the  air  is  moist,  the  guard  cells  absorb  moisture,  and  the 
result  is  the  widening  of  the  pore  and  any  excess  of  moisture 
in  the  interior  may  escape. 


Breathing  Pores.  63 

It  is  found  by  suitable  tests  that  the  outer  and  inner  faces  of 
the  guard  cells  are  thickened  and  cutinized,  while  the  radial 
walls  are  not  cutinized  and  very  thin. 

In  some  cases  epidermis  is  smooth,  but  in  the  majority  of 
cases  it  is  roughened  by  hairs  or  glands  and  the  walls  are  wavy 
in  outline.  The  number  and  distribution  of  stomata  varies 
greatly  in  different  epidermal  tissues.  They  are  found  prin- 
cipally on  the  under  surface  of  leaves.  In  some  leaves  many 
are  found  on  the  upper  surface,  in  others  none  at  all  are  found. 

Hold  a  leaf  of  the  common  trailing  garden  plant,  known  popu- 
larly as  the  Wandering  Jew,  against  the  side  of  a  large  cork, 
and  with  a  sharp  razor  cut  off  tangentially  little  patches  of  the 
epidermis,  both  from  the  upper  and  lower  surfaces.  Some  of 
the  underlying  green  cells  may  be  taken  with  the  epidermis, 
but  the  edges  of  the  sections  will  generally  consist  of  epidermis 
alone.  A  few  of  the  green  cells,  in  fact,  are  desirable,  as  they 
wilj  afford  an  opportunity  to  study  incidentally  chlorophyll 
granules.  • 

Mount  a  section  from  the  upper  surface  in  water  and  examine 
with  low  and  high  powers.  The  epidermal  cells  are  hexagonal 
in  shape,  transparent,  free  from  chlorophyll  granules  and  have 
no  intercellular  spaces  between  them.  With  iodine  solution 
the  protoplasm  stains  yellow  and  is  more  readily  seen.  In  all 
essentials  the  cells  are  like  those  of  the  onion  epidermis.  Note 
that  stomata  are  absent. 

In  the  same  manner  examine  a  section  from  the  lower  sur- 
face of  the  leaf.  The  epidermal  cells  are  pretty  much  the  same 
in  appearance,  but  scattered  among  them  are  numerous  sto- 
mata, easily  distinguished  by  the  pairs  of  crescentic  cells  with 
dense  contents  and  chlorophyll  granules.  These  cells  guard  an 
opening  or  pore  which  is  readily  seen.  With  iodine  solution 
the  granules  in  the  guard  cells  are  very  deeply  stained. 

Note  the  relative  sizes  of  epidermal  cells  and  guard  cells  and 
whether  the  latter  all  point  in  one  direction  or  not. 

In  some  of  the  sections  in  which  the  underlying  green  paren- 
chyma was  cut  with  the  epidermis  there  may  be  seen  long 
strands  of  spirally-marked  tubular  cells.  These  are  the  spiral 
wood  vessels  of  a  small  vein  of  the  leaf.  This  kind  of  vascular 
tissue  will  be  studied  in  a  later  exercise. 

Examine  the  epidermis  from  both  sides  of  a  leaf  of  the  Cul- 
tivated Lily  and  compare  with  that  of  the  Wandering  Jew. 
Note  the  absence  of  plant  hairs  from  both  leaves. 

Mount  fragments  of  a  Senna  leaf  in  a  few  drops  of  chloral 
hydrate  solution,  one  with  the  upper  surface  turned  up,  one 


64  Vegetable  Histology. 

with  the  lower  surface  turned  up  and  cover 
with  a  glass.  Heat  gently  till  the  liquid 
begins  to  boil  and  keep  it  at  that  tempera- 
ture for  a  few  moments,  then  cool,  add 
another  drop  of  liquid  if  necessary  and  ex- 
amine. 

The  leaf  has  become  sufficiently  trans- 
parent that  the  epidermal  cells,  stomata 
and  hairs  can  be  distinctly  observed.  The  fij?.  4i.— Epidermis  of 
epidermal  cells  are  polygonal  in  shape,  sto-  ^tdel-mS^  S*;  bi 
mata  are  present  on  both  surfaces  and  the  guard  ceiis  of  a 
hairs  are  one-celled  with  thick,  warty  walls,  phlJ?f  graiiu?es.^*^'^'^° 
often  curved. 

Often  the  epidermis  can  be  separated  by  warming  a  fragment 
of  a  leaf  in  a  solution  of  potassium  hydroxide  (about  2  per 
cent.)  until  it  boils,  cooling  and  pressing  the  cover  glass  firmly, 
while  at  the  same  time  giving  it  a  sliding  motion.  This  method 
may  be  tried  when  chloral  hydrate  solution  fails  to  clarify  suf- 
ficiently. 

Cross-sections  of  stomata  will  be  examined  in  the  exercises 
on  stems  and  leaves. 


CHAPTER   XIV. 

Epidermal  Appendages. 

All  the  outgrowths  or  appendages  of  the  surface  of  a  plant 
are  known  as  trichomes,  which  means  literally  hairs.  They 
consist  of  one  or  more  cells  usually  arranged  in  a  row  or  col- 
umn, sometimes  in  a  mass.  The  most  common  forms  of 
trichomes  are — 

1.  Hairs — these  are  the  principal  form. 

2.  Bristles — a  single-pointed  cell  or  row  of  cells,  much  thick- 
ened and  hardened. 

3.  Prickles,  like  bristles,  but  stouter. 

4.  Scales. 

5.  Glands — generally  short,  bearing  one  or  more  secreting 
cells. 

6.  Root-hairs,  long,  thin,  single-celled  and  subterranean. 

7.  Sporangia  of  ferns. 

8.  Ovules  of  flowering  plants. 

Trichomes  originate  mostly  from  the  growth  of  single  epi- 
dermal cells,  and  on  their  first  appearance  consist  of  slightly 
enlarged  and  protruding  cells.    These  may  elongate  and  form 


Epidermal  Appendages.  65 

single-celled  hairs,  which  may  be  simple  or  variously  branched. 
The  most  important  of  these  hairs  are  those  which  clothe  so 
abundantly  the  young  roots  of  most  of  the  higher  plants  and 
to  which  the  name  of  Root-hairs  has  been  applied.  These  are 
single  cells  which  have  very  thin  and  delicate  walls,  and  are 
the  active  agents  in  the  absorption  of  nutritive  matters  for  the 
plant.  On  the  above-ground  parts  the  hairs  frequently  have 
the  terminal  cell  developed  into  a  secreting  cell,  carrying 
gummy,  resinous  or  other  products.  Such  trichomes  are 
known  as  glandular  hairs. 

A  good  example  of  simple  hairs,  which  is  familiar  to  every 
one,  is  the  cotton  of  commerce.  Cotton  consists  of  the  hairs 
on  the  seeds  of  the  cotton  plant.  They  have  been  studied  in 
Chapter  II. 


Hairs  and  Glands  as  Found  on  a  Geranium. 

Make  cross-sections  of  the  young  stem  of  the  common 
Geranium,  which  need  not  necessarily  be  very  thin.  Mount 
several  in  water  and  examine  the  circumference  of  the  sections 
under  low  power.  Two  kinds  of  hairs  will  be  seen,  simple  and 
glandular. 

Under  high  power  the  simple  hairs  are  very  long,  consisting 
of  a  row  of  tapering  cells  which  contain  transparent  proto- 
plasm and  nucleus,  easily  seen  by  applying  iodine  solution. 
The  end  cell  is  long  and  pointed.  The  hair  fits  in  among  the 
epidermal  cells.  The  cell-walls  are  somewhat  thick  and  the 
cross  partitions  are  well  marked. 

There  are  three  kinds  of  gland  hairs. 

One  kind  is  rather  long,  consisting  of  a  stalk  of  about  six 
cells  (count  the  actual  number),  terminated  by  a  larger  gland 
cell,  which  is  round  and  full  of  contents.  The  two  or  three  cells 
just  below  the  gland  cell  are  suddenly  narrowed.  On  the  top 
of  the  gland  cell  there  is  a  crescent-shaped  glistening  mass 
which  is  found  to  lie  in  the  cell-wall  itself  between  the  outer 
and  inner  layers.  This  mass  dissolves  on  applying  alcohol  or 
ether,  which  indicates  that  it  is  probably  resinous.  Additional 
evidence  of  this  is  given  by  the  reaction  with  alcannin  solution, 
which  stains  the  mass  red.  The  resinous  matter  is  secreted  by 
the  gland  cell  and  forced  out  into  the  cell-wall  where  it  accumu- 
lates.    Iodine  solution  shows  the  presence  of  protoplasm  and 

nuclei  in  the  cells.  i     v,  i   „ic/^ 

Soak  some  sections  20  to  30  minutes  m  strong  alcohol,  also 


66 


Vegetable  Histology. 


in    alcannin    solution    and   note   the 
effect  upon  the  resinous  matter. 

The  other  two  kinds  of  ghmd-hairs 
are  very  short.  One  has  an  oblong 
gland  while  the  gland  cell  of  the 
other  is  round.  Note  the  number  of 
cells  in  the  stalk  of  each.  The  cells 
are  very  short. 

Note  which  of  the  three  kinds  is 
most  abundant. 

Submit  a  section  to  the  action  of 
chlor-zinc-iodine  solution  for  about 
half  an  hour  and  then  examine.  The 
outer  layer  of  the  walls  of  the  hair- 
cells  is  stained  brown,  showing 
cutinization ;  the  inner  portion  of  the 
walls  may  be  blue,  showing  cellulose 
substance. 

Try  the  clearing  effect  of  chloral- 
hydrate  solution  on  the  gland-hairs. 
The  contents  will  gradually  disappear,  leaving  the  cell-wall  dis- 
tinct, and  the  gland-cell  structure  may  now  be  studied. 

It  Is  thought  that  the  purpose  of  hairs  and  glands  is  to  afford 
protection  to  the  plant  against  insects,  etc.  The  odor  of  the 
Geranium  is  due  to  the  secretions  of  th6  gland-hairs. 


^m^M 


Hairs 


msr 


vr^<««c 


Fig.  42.— Hairs  on  Geranium 
stem.  1,  simple  tiair  ;  2,  3,  4, 
three  forms  of  gland  hairs 
(reduced,  from  Bastin). 


Hairs  on  Other  Plants. 


For  variety,  let  the  hairs  on  the  following  plants  be  studied : 

Clamp  a  piece  of  Mullein  leaf  between  pith  and  cut  several 

sections.     Mount  them  in  water  and  examine.     A  dense  growth 

of  branched  hairs  covers  the  leaf  and  beneath  these,  close  to 

the  surface,  will  be  found  a  few  short  glandular  hairs. 

Clamp  a  moderately  large  vein  (with  a  portion  of  the  leaf  on 
each  side)  of  Stramonium,  Digitalis,  Belladonna  and  Hyos- 
cyamus  leaf  respectively  in  pith  and  cut  cross-sections.  Exam- 
ine these  in  water  or  chloral  hydrate  solution.  Compare  the 
hairs  and  make  drawings.  These  are  leaves  of  four  important 
official  drugs  and  the  comparison  of  the  hairs  should  therefore 
be  of  special  interest. 


Starches.       ~  67 

CHAPTER    XV. 

Starches.  ' 

The  green  color  of  the  leaves  of  plants  is  due  to  a  colored  sub- 
stance called  chlorophyll,  which  is  diffused  through  certain 
proteid  granules  in  the  cells.  The  function  of  this  substance 
is  to  utilize  the  energy  of  the  sun's  rays  in  converting  carbon 
dioxide,  the  main  food  material  of  plants,  into  some  form  of 
carbohydrate.  Starch  is  a  carbohydrate,  but  that  formed  by 
the  green  coloring-matter  from  carbon  dioxide  is  not  starch. 
The  carbohydrate  in  question  is  combined  with  nitrogen  and 
sulphur  (taken  up  by  the  plant  in  the  form  of  salts)  into  a  pro- 
teid by  the  vital  processes  of  the  cells.  It  is  not  known  what 
the  composition  of  the  carbohydrate  is  nor  the  processes  in- 
volved in  building  up  the  proteid  substance. 

The  starch  granules  found  in  chlorophyll  bodies  were  at  one 
time  supposed  to  be  formed  directly  from  carbon  dioxide,  but 
Strasburger  has  clearly  shown  that  this  is  not  true.  They 
result  from  a  breaking  down,  by  the  chlorophyll  bodies,  of  proto- 
plasm previously  formed  by  those  bodies.  Starch,  hence,  is  a 
result  of  a  destructive  process.  It  is  probable  also  that  the 
starch  formed  by  amyloplasts  in  cells  devoid  of  chlorophyll  is 
also  formed  from  proteids. 

There  are  various  reserve  food  materials  found  in  the  plant, 
and  starch  is  one  of  the  most  important.  It  is  found  in  various 
parts  of  the  plant,  for  example,  the  stems  of  certain  palms, 
which  are  gorged  with  it ;  it  is  the  principal  substance  in  tap- 
roots, root-stocks,  corms,  bulbs,  tubers ;  many  fruits  and  seeds, 
as  grains,  pulses,  bananas. 

The  power  of  building  up  protoplasm  from  starch  is  possessed 
by  all  living  cells,  whether  possessing  chlorophyll  or  not,  and 
independent  of  sunlight,  but  no  new  carbohydrate  is  ever 
formed  without  light.  A  tuber  will  sprout  and  grow  in  the 
dark  until  all  the  starch  is  used  up,  when  growth  ceases,  and  to 
renew  growth  it  must  be  brought  into  light. 

Description  op  Starch  Grains. 

They  are  hard  and  of  various  sizes  and  often  possess  shapes 
and  markings  sufficiently  characteristic  to  identify  the  plant 
from  which  they  come.  They  vary  from  1  to  100  or  even  200 
microns.  A  micron  or  micromillimeter  is  one-thousandth  of  a 
millimeter  and  is  represented  by  the  sign  fi,  the  Greek  letter  m. 
Starch  gains  are  simple  or  compound.  Compound  grains  con- 
sist of  two  or  more  simple  grains  united  to  form  larger  grains. 
Potato,  wheat,  arrow-root,  corn  and  ginger  starch  are  examples 


68  Vegetable  Histology. 

of  simple  grains,  while  oat,  rice  and  colchicum  starch  are  com- 
pound. 

Nearly  all  starch  grains  possess  a  nucleus  or  Mlum,  around 
which  the  granule  is  built  up  in  layers,  which  differ  from  each 
other  in  transparency,  owing  to  different  amounts  of  water  in 
the  different  layers.  The  layers  are  concentric  or  eccentric 
according  as  the  nucleus  is  central  or  placed  to  one  side.  Exam- 
ples of  these  are  bean  and  potato  starch  respectively. 

Starch  grains  differ  from  one  another  in  a  number  of  ways, 
some  of  which  are  the  following : 

Size  and  shape  of  grains. 

Position  of  hilum  (central  or  eccentric). 

Number  and  distinctness  of  stratification  lines. 

Degree  to  which  the  hilum  is  fissured. 

Character  of  fissure. 

Potato  Starch. 

To  see  the  manner  in  which  starch  is  packed  in  the  cells  of 
the  various  plant  organs,  examine  cross-sections  of  the  potato 
tuber  and  podophyllum  rhizome. 

Cut  an  oblong  block  about  a  half-inch  square  on  the  end  from 
a  potato,  having  the  outer  coat  on  one  side.  Cut  thin  sections 
perpendicular  to  the  coat  and  mount  one  in  water.  Examine 
first  under  low  power  and  then  under  high  power. 

There  will  be  seen  on  the  exterior  a  layer  of  cork  cells  like 
rows  of  bricks.  Next  these  are  some  small,  closely  packed  par- 
enchyma cells,  rich  in  protoplasm,  but  containing  few  starch 
grains  and  these  are  small.  There  may  also  be  seen  cubical 
crystals  which  look  like  mineral  crystals,  but  are  proteid,  as 
they  stain  with  iodine  and  are  dissolved  by  caustic  potash. 
Farther  interior  the  cells  are  very  large  and  filled  with  large- 
sized  starch  grains,  and  the  cubical  crystals  are  wanting. 

Apply  dilute  iodine  solution.  The  grains  assume  a  dark 
blue  color,  the  protoplasm  stains  brown  and  the  cubes  the  same. 
It  will  be  observed  that  the  younger  starch  grains  in  the  outer 
small  parenchyma  cells  are  mostly  found  in  groups.  By  a  spe- 
cial method  of  preparation,  it  would  be  seen  that  the  grains  are 
grouped  around  a  mass  of  proteid  granules,  each  one  of  which 
is  attached  to  a  starch  gi'ain.  These  proteid  granules  are  the 
starch-builders  and  are  called  amyloplasts.  They  are  similar 
to  chlorophyll  bodies,  and  except  in  a  few  instances,  all  starch 
grains  are  formed  by  one  or  the  other  kind  of  these  bodies. 

Scrape  a  slice  of  potato  with  a  knife  and  mount  in  water. 
The  grains  are  ovate,  with  one  end  smaller  than  the  other  and 
a  hilum  or  nucleus  at  the  smaller  end.  The  hilum  is  sur- 
rounded first  by  concentric  lines  which,  farther  away,  become 
eccentric.     This  shows  that  at  first  the  growth  of  the  grain  was 


Starches.  69 

equal  on  all  sides,  but  afterwards  became  much  greater  on  one 
side  than  on  the  other.  This  view  is  borne  out  by  the  fact  that 
in  young  grains  the  hilum  is  central.  Some  of  the  lines  or  stria- 
tions  are  more  strongly  developed  than  others,  and  some  of  the 
grains  show  scarcely  any  lines.  Nearly  all  the  grains  are 
single,  but  some  double  grains  may  be  found  containing  two 
nuclei,  each  with  concentric  and  eccentric  markings  about  it 
and  a  distinct  dividing  line.  Some  grains  are  not  double,  but 
contain  two  nuclei,  i.  e.,  are  bi-nucleated. 

The  nucleus  is  usually  a  circular  spot  in  the  potato  grain, 
but  sometimes  it  is  fissured.  The  small  end  of  the  grain,  where 
the  nucleus  is  found,  is  thicker  than  the  broad  end,  the  grain 
being  shaped  somewhat  like  a  clam  shell.  To  observe  the  exact 
shape  of  the  grains  they  must  be  seen  from  all  sides.  This  is 
readily  done  by  pressing  gently  on  the  cover  glass  with  a  needle, 
when  the  grains  will  roll  over  and  may  be  seen  in  various  posi- 
tions. Another  way  to  cause  the  grains  to  roll  is  to  place  a 
drop  of  alcohol  at  the  edge  of  the  cover  glass,  which,  by  mixing 
with  the  water,  causes  currents.  When  thus  observed,  the 
grains  are  seen  to  be  somewhat  flattened  (Fig.  43,  II). 

Let  a  drop  of  5  per  cent,  caustic  potash  solution  run  under 
the  cover  glass  and  watch  the  starch  grains  as  the  alkali  comes 
in  contact  with  them.  They  swell,  and  at  first  the  layers  be- 
come more  distinct,  but  after  a  while  they  grow  less  distinct 
and  finally  disappear.  The  more  watery  layers  at  first  absorb 
water  under  the  action  of  the  potash  more  rapidly,  and  thus 
stand  out  more  distinctly.  Finally  the  whole  grain  dissolves 
and  disappears.  Other  reagents  which  cause  starch  to  swell 
and  dissolve  are  concentrated  solutions  of  chloral  hydrate,  zinc 
chloride,  calcium  chloride,  strong  hydrochloric  and  sulphuric 
acids,  oil  of  cloves. 

Mount  some  potato  starch  in  water  and  apply  a  gentle  heat 
near  the  edge  of  the  cover  glass  until  the  grains  there  become 
translucent,  then  quickly  remove  the  flame  and  cool  the  slide. 
Compare  the  grains  that  have  been  affected  by  heat  with  those 
that  have  just  begun  to  change  and  those  that  are  still  intact. 
Heat  causes  the  grains  to  swell  and  ultimately  to  assume  a 
gelatinous  form.  Intermediate  granular  and  translucent 
stages  occur  in  those  grains  that  have  not  been  heated  too 
strongly.  The  temperature  of  gelatinization  varies  appreciably 
with  the  kind  of  starch. 

Examine  potato  starch  mounted  in  glycerin,  also  in  oil  of 
cloves.  The  grains  appear  brighter  and  the  lines  are  almost 
invisible.  If  an  air  globule  be  imprisoned  in  the  hilum  it  will 
appear  as  a  dark  spot.  Glycerin  and  oil  of  cloves  and  any 
similar  liquids  are  unsuited  as  a  medium  for  mounting  starch 
because  of  their  high  refractive  power,  which  causes  details  of 
structure  to  be  almost  invisible. 


70  Vegetable  Histology. 

Other  Starches. 

Maranta. — A  large  proportion  of  the  edible  starches  obtained 
from  the  rhizomes  or  root-stocks  of  various  plants  are  known 
in  commerce  under  the  name  of  arrow-root.  Properly  the  name 
should  be  restricted  to  the  starch  yielded  by  two  or  three  species 
of  Maranta,  the  chief  of  which  is  Maranta  arundinacea.  Ac- 
cording to  the  country  from  which  it  is  derived,  maranta  starch 
is  known  as  Bermuda,  St.  Vincent,  West  Indian  or  Natal  arrow- 
root. The  grains  are  simple  and  mostly  ovoid,  the  largest  ones 
being  marked  by  fine  striations,  but  less  distinct  than  those  of 
potato  starch.  (The  nucleus  is  rounded,  linear  or  star-shaped 
and  usually  placed  eccentrically.  The  grains  are  rather  large, 
but  smaller  than  potato  starch. 

Tous-LES-Mois^  Tulema  or  Queensland  arrow-root  is  obtained 
from  several  species  of  canna,  a  genus  closely  allied  to  maranta. 
The  grains  resemble  those  of  potato  starch,  but  are  much  larger. 

Curcuma  or  East  Indian  arrow-root  is  obtained  from  the 
root-stocks  of  several  species  of  the  genus  Curcuma  (Zingi- 
beracese)  chiefly  Curcuma  angustifolia.  The  grains  are  ovate 
and  taper  to  a  nipple-like  projection  at  one  end,  in  which  the 
hilum  is  located.  The  grains  are  simple,  rather  large  and  so 
flat  that  on  edge  they  seem  rod-shaped  (Fig.  43,  VIII).  Ginger 
starch  is  shaped  somewhat  like  that  of  curcuma. 

Brazilian  arrow-root,  cassava,  or  tapioca  of  commerce,  is 
manufactured  from  the  starch  obtained  from  the  tubers  of 
Manihot  utilissima.  Most  of  the  grains  are  blurred,  which  is 
due  to  the  heating  in  the  process  of  manufacture,  but  many 
grains  may  be  found  uninjured.  Soak  some  of  the  starch  a 
few  hours  in  water,  mash  a  little  out  on  at  slide  in  water,  cover 
and  examine.  The  unchanged  grains  appear  as  in  Fig.  43,  IX; 
the  others  are  more  or  less  gelatinized. 

.  British  arrow-root  is  potato  starch,  which  is  sometimes  sold 
under  this  name.  The  French  excel  in  the  preparation  of  imi- 
tations of  the  more  costly  starches  from  potato  starch.  Its 
chief  use,  however,  as  an  edible  starch,  is  for  adulterating  other 
more  costly  preparations.  It  can  easily  be  distinguished  under 
the  microscope. 

Sago^  or  pearl  sago,  of  commerce  is  obtained  by  heating  and 
stirring  the  moist  starch  of  the  sago  palm,  Metroxylon  Sagu. 
As  in  the  case  of  tapioca,  many  of  the  grains  are  gelatinized, 
but  some  are  still  intact,  which  appear  as  in  Fig.  43,  X. 

Maize  or  corn  starch  is  obtained  from  the  fruits  of  Zea  Mays. 
The  grains  are  simple,  rounded  or  polygonal  and  tolerably  uni- 
form. The  hilum  appears  as  a  point  or  more  often  as  a  stellate 
cleft,  less  frequently  as  a  large  central  cavity.  This  starch  ia 
one  of  the  most  frequently  used  adulterants  of  powdered 
articles,  and  hence  should  be  closely  studied. 


Starches. 


71 


Wheat  starch  is  obtained  from  several  species  of  Triticum. 
It  consists  of  large  rounded  grains  and  numerous  small  ones! 
Hilum  and  striations  are  seldom  visible.  The  grains  are  oval 
or  concavo-convex  in  shape.  Wheat  starch  is  also  used  as 
adulterant. 

Rice  starch  comes  from  Oryza  sativa,  and  mav  be  examined 
after  soaking  for  three  hours  or  more  in  water.     The  grains  are 


^£^^^  wMi^o   ^l?'3i 
o^m^  ^<^me»  ^ei%4 


Fig.  43.— Starches. 

I,  Wheat;   II,  Potato  ;   III,  Arrow-root ;    IV,  Corn  ;   V,  Oat ;   VI,  Rice  ;   VII,  Bean; 
VIII,  Curcuma  ;   IX,  Tapioca  ;  X,  Sago  ;   XI,  Sarsaparllla  ;   XII,  Euphorbia. 

very  small  and  angular,  with  no  hilum.  The  small  grains 
result  from  the  breaking  apart  of  larger  compound  oval  grains, 
some  of  which  may  be  present  in  the  mount. 

Oat  starch  from  Avena  sativa  resembles  rice  starch  very 
much.  Some  of  the  granules  are  rounded,  semi-circular  or 
lemon-shaped,  which  helps  to  distinguish  it  from  rice  starch 
(Fig.  43,  V  and  VI). 


72  Vegetable  Histology. 

CHAPTER    XVI. 

Aleurone  Grains. 

Protoplasm  exists  both  as  active  and  inactive.  In  the  active 
state,  as  found  in  actively-growing  cells,  it  exhibits  vital  phe- 
nomena in  a  marked  degree,  but,  as  found  in  the  cells  of  seeds, 
tubers  and  thickened  roots,  it  exhibits  few  signs  of  vitality, 
contains  comparatively  little  water  and  its  condition  approxi- 
mates that  of  a  solid. 

Ordinary  protoplasm  is  formless;  even  under  the  highest 
powers  it  exhibits  no  structure  except  the  presence  of  numer- 
ous very  minute  granular  bodies  called  microsomes,  the  nature 
and  uses  of  which  are  not  yet  understood.  It  passes,  however, 
into  several  modifications  which  exhibit  a  more  or  less  charac- 
teristic structure.  The  most  important  of  these  is  the  chlor- 
ophyll body.  The  amyloplasts,  spoken  of  under  starch,  are 
another  form  with  structure.  These  are  active  forms  of  proto- 
plasm. 

Alerone  grains  are  a  structural  form  of  inactive  protoplasm, 
the  use  of  which  is  to  act  as  a  reserve  food  material.  Plants 
lay  up  a  store  of  food  in  various  forms,  one  of  which  we  have 
studied,  i.  e.,  starch.  Other  non-proteid  food  materials  are  oil, 
inulin,  sugar.  Some  albuminous  forms  are  also  stored  up,  the 
most  important  of  which  are  aleurone  grains.  These  are  found 
chiefly  in  seeds  and  most  abundantly  in  oily  ones.  They  are 
usually  rounded  granules,  often  very  small,  but  sometimes  quite 
large,  as  in  Croton  and  Castor  seeds  and  in  the  Brazil-nut.  In 
some  cases  the  granules  appear  homogeneous  in  structure,  as 
in  peony,  almond,  cherry  and  apple  seeds;  in  other  cases  they 
contain  various  substances,  as  oily  matters,  mineral  crystals 
and  crystalloids,  as  in  Brazil-nut,  pumpkin  and  castor  seeds, 
walnut. 

The  ground  substance  of  aleurone  grains  is  dissolved  or 
attacked  by  water,  hence  the  latter  is  an  unsuitable  mounting 
medium.  Alcohol,  glycerin  or  fixed  oils  have  no  solvent  action 
on  the  grains.  Dilute  alkalies  readily  dissolve  the  aleurone 
grains  except  the  mineral  matter  in  them. 

Castor  Seed. 

Remove  the  hard  seed-coat  and  make  thin  sections  of  the 
endosperm.  Mount  one  in  strong  glycerin  and  examine  with 
low  and  high  powers.  Not  much  wull  be  made  out  with  low 
power.  The  cells  will  be  seen  to  be  crowded  with  rounded 
granules,  looking  much  like  starch. 

Under  high  power,  rounded  or  ellipsoid  bodies,  imbedded  in  a 
finely  granular  matter  fill  the  cells.     These  are  the  aleurone 


Chloroplasts.  73 

grains.  After  a  little  time  the  clearing  effect  of  the  glycerin 
shows  that  the  grains  are  not  homogeneous,  but  contain  a 
denser,  polygonal  body  looking  like  a  crystal  and  known  as  a 
crystalloid  which  varies  in  size,  being  often  nearly  as  large  as 
the  grain.  Lying  alongside  of  this  is  seen  a  globular  body, 
strongly  refractive  and  composed  of  magnesium  and  calcium 
phosphates  and  known  as  a  globoid  (Fig.  44). 

Add  strong  iodine  solution  to  the  section  in  glycerin.  The 
grains  stain  brown,  especially  deep  in  the  crystalloid,  indicat- 
ing the  proteid  nature  of  the  latter.  The  globoids  remain 
unstained. 

There  is  no  blue  coloration,  showing  the  absence  of  starch. 

Mount  a  fresh  section  in  water  and  watch  the  aleurone  grains. 
The  ground  substance  soon  swells  and  dissolves,  leaving  the 
crystalloid  standing  out  more  distinct.  After 
a  time  this  also  swells  and  loses  its  angles  and 
finally  disappears.  Oil  globules  may  also  be 
seen  to  collect  and  run  out  from  the  ruptured 
cells.  These  have  a  refractive  appearance, 
being  bounded  by  a  dark  band. 

If  fresh  sections  be  mounted  in  alcannin  or 
cyanin    solution   the   oil    globules   will   stain 
after  a  time  deep  red  or  blue.    Oil  is  a  reserve    ''*|rain^s7n^S**of 
food  for  the  plant,  like  starch  and  aleurone,      ?^^*°'*„l®®^;  1?°X 

■M   .     J.  -,  '  1  -1  -,  ^^S      crystallolas 

and  is  found  m  a  great  many  seeds  and  spores,  and  globoids 
often  in  large  quantity.  (Sachs). 

Mount  a  section  in  dilute  alkali  (about  1  per  cent.)  and  watch 
a  grain  closely.     All  parts  dissolve  except  the  globoid. 


CHAPTER    XVII. 

Chloroplasts  or  Chlorophyll  Corpuscles. 

When  speaking  of  starch,  it  was  noted  what  importance  these 
bodies  are  to  the  life  of  plants,  and  that  without  them  plants 
could  not  exist.  They  are  the  most  important  structural  form 
of  protoplasm.  They  are  the  bodies  which  give  the  green  color 
to  plants  and  are  commonly  rounded,  oblong  or  flattened  in 
shape.  An  odd  form  is  the  spiral  bands  in  Spirogyra  (which 
see). 

Chloroplasts  are  proteid  in  nature,  having  the  power  of 
growth  and  division,  and  always  closely  associated  with  ordin- 
ary protoplasm,  and  are  hence  to  be  regarded  as  part  of  the  liv- 
ing protoplasm. 

In  many  plants  chloroplasts  are  small  and  difficult  to  study, 
but  in  others  they  are  easily  studied.     Most  any  moss,  the 


74  Vegetable  Histology. 

prothallia  of  ferns,  Eel-grass,  Water-weed,  are  excellent  objects 
of  study. 

Mount  a  few  fresh  leaves  of  a  moss  in  water  and  examine  the 
cells  lying  near  the  edge  of  the  leaf.  The  cells  contain  numer- 
ous rounded  greenish  bodies — the  chlorophyll  bodies.  Note 
the  closely-packed,  rectangular-shaped,  somewhat  thick-walled 
cells.  Try  to  observe  chlorophyll  bodies  that  are  constricted 
in  the  middle.     Such  are  in  the  act  of  division. 

Run  strong  alcohol  under  the  cover  glass.  The  green  bodies 
are  slowly  bleached,  the  chlorophyll  is  dissolved  out  and  forms  a 
greenish  solution  in  the  cells.  From  this  experiment  it  is  con- 
cluded that  the  bodies  consist  of  two  parts,  a  proteid  ground 
work,  through  which  is  diffused  the  chlorophyll  or  green  color 
substance. 

Add  iodine  solution  to  a  leaf  that  has  been  bleached  with 
alcohol.  The  transparent  protoplasm  of  the  cells  is  now 
stained  brown  and  made  visible.  The  ground  substance  of  the 
chloroplasts  also  stains  brown,  indicating  a  proteid  substance. 

Starch  grains  in  chloroplasts.  Put  some  leaves  of  a  vigor- 
ously-growing moss  that  has  been  exposed  to  sunlight  for  two 
or  three  hours  in  alcohol  until  bleached.  Then  mount  a  leaf 
in  water,  focus  it  and  apply  a  drop  of  chloral-hydrate  iodine 
solution  at  the  edge  of  the  cover  glass  and  watch  closely  the 
chlorophyll  body  as  the  reagent  comes  in  contact  with  it.  The 
ground  substance  is  seen  to  swell  rapidly  and  becomes  trans- 
parent, leaving  the  starch  grains  stained  blue,  visible.  The 
latter  are  small  and  elongated.  By  the  continued  action  of  the 
chloral  the  whole  structure  gradually  disappears.  The  starch 
that  is  formed  in  the  chlorophyll  bodies  in  the  daytime  is  dis- 
solved during  the  night  and  transferred  to  other  parts  of  the 
plant. 

CHAPTER    XVIII. 

Secretion  Sacs^  Intercellular  Air  Spaces  and  Secretion 

Reservoirs. 

Some  cells  at  maturity  lose  their  protoplasm  and  their  proper 
cell  character  and  become  filled  with  secreted  matters.  These 
form  the  secretion  sacs.  They  are  of  various  forms,  but  more 
commonly  resemble  parenchyma  cells  in  appearance  and  char- 
acter of  their  walls.  Sometimes  they  are  much  elongated  and 
resemble  latex  tissue.  The  sacs  are  given  names  according  to 
the  secretion  they  contain,  thus,  resin  sacs,  mucilage  sacs,  etc. 

Intercellular  air  spaces  are  more  or  less  abundant  in  nearly 
all  multicellular  plants,  their  probable  function  being  to  supply 
air  to  the  interior  tissues  for  respiratory  purposes.  In  aquatics 
the  spaces  are  usually  large  and  often  regular  in  shape,  while 


Air  Spaces  and  Secretions.  75 

in  most  terrestrial  plants  they  are  small  and  angular.  Air 
spaces  are  of  two  kinds,  schizogenous  and  lysigenous.  The 
former  are  formed  by  the  splitting  of  the  cell-wail  common  to 
two  or  three  cells.  The  latter  result  from  the  breaking  down 
of  some  cells,  leaving  a  space.  Secretion  reservoirs  or  canals 
and  intercellular  air  spaces  differ  from  each  other  only  in  their 
contents,  the  former  containing  resins,  gums,  oleo-resins,  etc., 
the  latter  only  air. 

Intercellular  Air  Spaces  and  Resin  Sacs. 

Calamus  (Sweet  Flag)  Rhizome. — Make  thin  cross-sections 
of  the  fresh  rhizome  or  of  the  dried  one  after  having  soaked  it 
in  water  to  soften  it.  Mount  a  section  in  water  and  examine 
the  outer  third  of  the  section.  The  parenchyma  cells  are  loosely 
arranged  in  chains  with  large  and  tolerably  regular  intercellular 
air  spaces.  Most  of  the  cells  are  filled  with  starch  grains, 
together  with  protoplasm  and  nucleus,  but  scattered  here  and 
there  among  these   are   larger  spherical   cells  filled   with   a 


Fig  45.— Part  of  cross-section  of  Calamus  rhizome,  a,  volatile  oil  and  resin  gao ; 
b,  sacs  with  transparent  secretion  ;  c,  starchy  parenchyma  cells ;  d,  air  spact 
(Bastin). 

refractive  or  glistening  material,  sometimes  intermixed  with  a 
brownish  solid.  These  are  the  secretion  sacs  (Fig.  45).  The 
refractive  contents  of  the  sacs  are  not  saponified  by  caustic 
potash,  which  fact  indicates  a  volatile  oil. 

Further  evidence  in  favor  of  this  conclusion  is  given  by 
applying  cyanin  solution  to  a  section,  when  after  some  time  the 
contents  are  stained  blue.  The  solid  brown  matter  found  in 
some  of  the  cells  is  resin  and  also  stains  blue  with  cyanin. 


76  Vegetable  Histology. 

Ginger  Rhizome. — Cut  cross-sections  from  the  dry  rhizome, 
mount  a  thin  one  in  water  and  examine  with  low  power  first. 
The  section  is  made  up  for  the  most  part  of  large  starch  grains 
in  ordinary  parenchyma  cells,  but  here  and  there  may  be  seen 
groups  of  thick-walled  light  yellow  cells,  the  fibres  and  vessels 
of  the  wood  bundles.  In  addition  to  these  there  will  be  seen 
more  or  less  distinctly  lighter  or  darker  yellowish  homogeneous 
masses.  They  are  the  oleo-resinous  contents  of  secretion  sacs 
which  are  scattered  over  the  whole  section.  If  dilute  iodine 
solution  be  added  the  section  turns  dark,  but  the  groups  of 
woody  cells  now  stand  out  more  conspicuously  in  contrast  by 
their  deep  yellow  walls  and  the  resin  sacs  also. 


Fig.  46.— Cross-section  of  Ginger  rhizome  showing  a  vascular  bundle  and  paren- 
chyma;   oil,  oleoresin  sacs;    scl.  f.,  bast  fibres;   v,  wood  vessel  (Vogl). 

To  remove  the  starch  grains  which  interfere  by  their  great 
numbers  mount  a  section  in  chloral  hydrate  solution  and  warm 
gently.  The  shape  of  the  parenchyma,  wood  bundles  and  the 
secretion  sacs  can  now  be  seen  more  easily. 

Mount  a  section  in  caustic  alkali  (5  per  cent.).  The  starch 
dissolves  and  the  secretion  sacs  turn  a  deep  reddish-brown, 
because  of  the  union  of  alkali  with  the  resin.  In  the  liquid  of 
the  sacs  unchanged  oil  drops  are  visible.  Use  high  power  also 
in  the  above  experiments  and  also  study  the  starch  grains 
carefully. 

Mucilage  Sacs  in  Marsh-mallow  Root. 

Wrap  a  piece  of  the  root  in  wet  filter  paper  until  it  is  soft 
enough  to  cut.     Make  cross-sections  and  place  them  in  dilute 


Air  Spaces  and  Secretions. 


77 


Mount 


alcohol  (not  water,  because  it  acts  on  the  mucilage) 
a  section  in  dilute  alcohol  or 
glycerin.  Smaller  cells 
densely  filled  with  starch 
constitutes  the  greatest  pro- 
portion of  the  section.  Scat- 
tered irregularly  among 
these  are  numerous  larger 
rounded  sacs  containing  a 
transparent  substance — the 
mucilage  sacs. 

Add  a  drop  of  iodine  so- 
lution to  a  fresh  section. 
Most  of  the  cells  turn  dark 
(starch),  but  the  mucilage 
sacs  remain  as  before.  In 
the  large  central  area 
groups  of  yellow-stained 
wood  bundles  are  visible, 
while  in  the  cortical  layer 
groups  of  similarly  stained 
bast  fibres  are  seen.  A  dis- 
tinct circle  of  small  cells, 
free  from  starch,  divides  the 
cortical  layer  from  the 
larger  central  area.     It  is  the  cambium  zone. 

Remove  nearly  all  of  the  liquid  from  the  iodine  section  and 
add  a  drop  or  two  of  sulphuric  acid  (diluted  with  one-fourth 
water).  The  mucilage  is  stained  a  deep  brown  and  the  walls 
blue  (cellulose).  Under  favorable  conditions  delicate  con- 
centric lines  may  be  seen  in  the  mucilage. 

Soften  a  piece  of  Elm  bark  and  examine  cross-sections  in  the 
same  manner  as  in  the  case  of  marsh-mallow  root.  It  has  large 
and  numerous  sacs. 


Fig.  47.— Cross-section  of  Althaea  root,  sch, 
mucilage  sacs  ;  c,  cambium  ;  b,  bast  fibres ; 
lb,  a  bundle  of  wood  vessels  and  fibres ;  m, 
medullary  rays  (Tschirch). 


Mucilage  and  Raphide  Sacs  in  Squill  Bulb. 

Crystals  are  of  such  general  occurrence  in  widely  different 
orders  of  the  higher  plants  that  there  are  perhaps  none  in 
which  they  may  not  be  detected.  They  have  been  found  in 
nearly  all  parts  of  the  vegetable  structure,  more  commonly  in 
the  interior  of  parenchyma  cells,  sometimes  in  specialized 
crystal  cells. 

They  occur  either  singly  or  in  groups.  The  most  common 
forms  are  the  octahedron  and  the  prism.  Sometimes  the  crys- 
tals are  much  elongated  and  pointed,  like  a  needle,  and  are  then 
known  as  raphides  (from  raphis,  a  needle).     These  are  gener- 


78  Vegetable  Histology. 

ally  massed  in  a  compact  bundle,  like  a  wheat-sheaf,  occupying 
a  large  part  of  the  interior  of  the  cell.  Raphides  are  by  no 
means  of  such  general  occurrence  as  ordinary  crystals,  being 
restricted  to  certain  orders. 

Soak  a  slice  of  the  scale  of  a  squill  bulb  in  water  just  long 
enough  to  soften  it  and  then  place  in  a  mixture  of  alcohol  and 
glycerin  6-8  hours.  Make  thin  sections  and  mount  in  glycerin. 
The  tissue  is  composed  of  large,  clear,  typical,  parenchyma  cells, 
with  here  and  there  a  cell  filled  with  long  needle  crystals, 
together  with  mucilage. 

Apply  a  drop  of  acetic  acid.  The  needles  do  not  dissolve. 
To  another  section  add  hydrochloric  acid;  the  needles  dissolve 
without  effervescence,  showing  that  they  are  calcium  oxalate. 
Had  they  been  carbonate  they  would  have  dissolved  in  acetic 
acid  with  effervescence. 

Secretion  Reservoirs. 

These  differ  from  intercellular  air  space  only  in  being  filled 
with  secretions  instead  of  air.  They  differ  from  secretion  sacs 
in  that  they  are  spaces  surrounded  by  a  number  of  cells,  while 
the  sacs  are  single  cells.  The  reservoirs  are  often  merely  irreg- 
ular spaces  left  by  the  breaking  down  of  one  or  more  cells,  but 
they  sometimes  have  a  remarkable  regularity  of  form  and  clear- 
ness of  outline. 

It  has  been  observed  that  these  spaces  are  not,  as  a  rule,  met 
with  in  plants  having  the  simple  secretion  sacs.  The  cells 
which  surround  the  more  complete  cavities  are  quite  different 
from  the  other  parenchyma  cells,  and  they  are  collectively 
called  the  epithelium  of  the  spaces.  These  are  the  secreting 
cells  and  the  secreted  matters  are  discharged  in  some  manner 
into  the  reservoirs,  where  they  accumulate. 

Cut  cross-sections  of  a  pine  needle  between  pith  and  mount 
in  water  or  chloral  hydrate  solution.  Arranged  around  the 
section  near  the  peripher^^  are  five  or  six  cavities,  each  having 
a  lining  epithelium  of  flattened,  thin-walled  cells,  and  next  to 
these  is  a  circle  of  glistening,  very  thick-walled  cells  (Fig.  62). 
These  structures  are  channels  which  carry  the  turpentine 
secretion.  Also  make  cross-sections  of  the  stem  of  a  softened 
clove  bud  and  mount  in  water.  Near  the  circumference  will 
be  found  a  double  row  of  large  cavities,  each  lined  with  a  mem- 
brance  of  secreting  cells.     The  cavities  carry  the  "oil  of  cloves.'^ 


Fibres.  79 

CHAPTER   XIX. 

Wood  (Libriform)  Fibres  and  Bast  (Liber)  Fibres. 

These  fibres  belong  to  the  prosenchyraatous  series  of  tissues. 
This  series  embraces  those  cells  which  at  maturity  lose  their 
nuclei  and  protoplasm,  and,  therefore,  their  distinctively  cellu- 
lar character,  and  have  their  walls  thickened  by  secondary 
deposits.  They  sometimes  contain  starch  and  traces  of  pro- 
teid  matter,  but  take  no  active  part  in  the  nutritive  processes 
of  the  plant.  They  serve  it  mainly  for  strengthening  or  sup- 
porting, and  hence  have  been  called  mechanical  tissues.  They 
are  serviceable  also  in  conducting  the  sap.  The  elements  of 
these  tissues  are,  for  the  most  part,  elongated  and  oblique-ended 
or  taper-pointed.  Among  the  shorter  cells  transitions  occur. 
Between  them  and  sclerotic  parenchyma,  and  between  the 
fibrous  forms  and  collenchyma,  every  gradation  may  be  found. 

Bast  and  wood  fibres  are  found  in  the  so-called  fibro-vascular 
bundles  of  the  plant.  These  bundles  constitute  the  fibrous 
framework  of  the  plant,  corresponding  somewhat  to  the  bony 
skeleton  of  the  human  body.  In  the  leaf  they  are  the  system  of 
veins,  and  in  the  stem  and  root  the  tough  resistant  portions. 
Their  function  is  partly  to  give  strength  and  partly  to  conduct 
the  fluids  of  the  plant.  The  cells  composing  the  bundles,  there- 
fore, for  the  most  part,  have  their  walls  thickened  and  are 
elongated  in  the  direction  of  the  length  of  the  organ  bearing 
them.  They  belong  chiefly  to  the  prosenchymatous  series, 
although  other  tissues  are  commonly  included  in  the  bundles. 
In  some  plants,  as  the  stem  of  the  Indian  Corn  and  the  petiole 
of  the  Plantain,  the  bundles  may  be  readily  separated  in  the 
form  of  tough,  stringy  masses  from  the  softer  surrounding 
tissue.  (Break  the  petiole  of  a  Plantain  leaf  and  note  the 
tough  threads  protruding  from  the  broken  end.  These  are  the 
bundles). 

Although  a  number  of  kinds  of  tissues  are  usually  found  in 
the  fibro-vascular  bundles,  only  two  kinds  are  really  essential, 
namely,  ducts  (and  tracheids,  which  may  be  regarded  as  im- 
perfectly-formed ducts)  and  sieve  cells.  These  and  their  asso- 
ciated tissues  always  constitute  separate  longitudinal  portions 
of  the  bundle.  The  nature  of  ducts  and  sieve  cells  will  be  ex- 
plained later.  The  portion  of  the  bundle  to  which  the  ducts 
belong  is  called  the  xylem,  which  means  the  wood,  and  that  to 
which  the  sieve  cells  belong  is  called  the  phloem  or  hast  (phloem 
means  hark).  The  reason  for  calling  this  the  bast  or  bark  is 
the  fact  that  the  inner  bark  of  gymnosperms  and  dicotyledons, 
also  called  the  bast  layer,  is  composed  of  the  phloem  portions 
of  the  bundles,  which  are  arranged  in  a  circle.     Within  these 


80  Vegetable  Histology. 

are  the  xylem  portions  of  the  bundles,  forming  the  cylinder  of 
wood. 

The  term  bast  was  originally  given  to  the  inner  bark  of  the 
Linden  tree,  which  was  called  Bass-wood  tree.  It  is  now  ap- 
plied to  the  inner  layer  of  any  bark.  The  term  liber,  which  is 
also  given  to  the  inner  or  bast  layer,  was  applied  in  a  more  gen- 
eral way  to  any  smooth,  inner  bark  upon  which  one  could 
write  (liber  is  the  Latin  for  book) . 

The  peculiar,  long,  tough,  thick-walled  cells  which  impart 
toughness  to  the  inner  bark,  making  it  valuable  in  the  arts,  are 
hence  known  as  bast  or  liber  fibres.  It  is  in  the  bark  of  dicoty- 
ledons that  liber  fibres  occur  most  abundantly  in  the  phloem 
portions  of  the  bundles. 

Some  plants,  for  example,  the  monocotyls,  have  no  true  bark, 
consequently  the  bast  or  phloem  portions  of  the  bundles  do  not 
lie  in  the  inner  or  bast  layer  of  the  bark,  and  the  bast  fibres  of 
such  bundles  are  apparently  misnamed.  However,  the  term 
liber  or  bast  fibres  has  been  extended  to  embrace  all  those  fibres 
that  occur  in  the  phloem  portions  of  the  bundles,  whether  they 
occur  in  the  inner  bark  or  elsewhere  in  the  plant  and  whether 
they  occur  in  gymnosperms,  dicotyls  or  monocotyls. 

The  term  libriform  has  reference  to  the  general  resemblance 
of  the  wood  fibres  to  bast  or  liber  fibres.  Wood  fibres  usually 
differ  from  liber  fibres  in  being  relatively  less  elongated,  less 
tough  and  flexible  and  less  strongly  lignified  at  maturity,  but 
there  are  many  exceptions,  especially  in  monocotyls,  where  the 
two  tissues  are  often  indistinguishable  by  structure  alone.  The 
fibres  vary  often  in  length. 

Wood  and  bast  fibres  may  be  conveniently  studied  in  a  mod- 
erately stout  Geranium  stem,  in  which  they  have  already  been 
noted  superficially  in  Chapter  IX.  Make  thin  cross-sections 
of  a  Geranium  stem  and  study  without  staining  and  also  by 
staining  with  phloroglucin  and  hydrochloric  acid  (red). 

The  wood  circle  lies  within  the  cambium  zone  and  surrounds 
a  central  area  of  parenchyma  cells,  the  pith.  It  is  composed  of 
the  xylem  portions  of  numerous  bundles  which  have  grown 
together  into  a  solid  ring  of  woody  tissue.  The  wood  fibres,  in 
cross  section,  are  seen  to  be  very  thick-walled,  compactly 
arranged,  more  or  less  compressed  laterally  by  mutual  pressure, 
so  as  to  appear  angular.  They  lie  next  to  the  cambium  zone. 
The  cells  are  separated  by  a  distinct  line,  the  middle  lamella. 
The  walls  are  lignified,  as  shown  by  the  red  stain,  the  middle 
lamella  being  deeper  red  than  the  rest  of  the  walls.  There  are 
no  intercellular  spaces.  The  cells  are  unequal  in  size  and 
irregularly  arranged.  There  are  delicate  stratification  lines 
and  occasionally  pore-canals  in  the  walls,  though  these  are  seen 
with  diflSculty. 


Fibres. 


81 


In  the  wood  ring  there  are  other  cells  besides  those  just 
described,  with  much  larger  openings,  the  ducts  or  tracheids, 
which  will  be  described  a  little 
later.  Wood  fibres  do  not  oc- 
cur in  all  flbro-vascular  bun- 
dles, but  are  nearly  always 
present  in  woody  and  herbace- 
ous dicotyls. 

Bast  Fibres. — Outside  the 
cambium  zone  is  another  cir- 
cle of  thick-walled  cells,  look- 
ing in  all  respects  very  much 
like  the  wood  fibres  just  de- 
scribed. These  are  the  bast  or 
liber  fibres.  The  description 
of  the  wood  fibres  answers  also 
for  the  bast  fibres.  With 
phloroglucin  and  hydrochloric 
acid  they  stain  red  and  the  mid- 
dle lamella  deepest.  Stratifi- 
cation lines  and  pore  canals 
can  also  be  seen  with  care.  In 
many  cases  bast  fibres  are  not  as  strongly  lignified  as  wood 
fibres  and  take  the  stains  less  deeply.  In  such  cases  the  ligni- 
fication  is  chiefly  confined  to  the  outer  layers  of  the  cell-walls, 
while  the  inner  layers  are  more  or  less  cellulose  in  nature ;  with 
double  stains,  as  methyl-green  and  eosin  the  outer  layers  will 
stain  green  and  the  inner  ones  reddish. 

Bast  fibres  are  confined  not  alone  to  the  phloem  or  bast  por- 
tion of  fibro-vascular  bundles,  but  are  often  found  in  other  por- 
tions of  the  plant,  for  example,  the  strengthening  fibres  beneath 
the  epidermis  of  some  leaves ;  in  the  ground  tissue  of  many  vas- 
cular cryptogams.  As  a  rule,  bast  fibres  are  thicker-walled 
than  wood  fibres,  but  both  kinds  may  vary  considerably  in  the 
thickness  of  the  walls,  in  their  lengths  as  compared  with  their 
diameters,  in  the  number  of  pore  canals. 


Fig.  48.— Portion  of  cross-section  of 
wood  ring  of  Geranium  stem,  w, 
wood  fibres ;  b,  duct ;  c,  middle 
lamella  (Bastin). 


Wood  and  Bast  Fibres  in  Longitudinal  Section. 

Treat  several  longitudinal  radial  sections  of  the  Geranium 
stem  with  Schulze's  macerating  fluid,  as  directed  under  this 
reagent.  Wash  them  in  water  and  place  them  in  very  dilute 
methyl-green  solution  for  a  long  time,  say  24  hours  or  more, 
which  will  stain  only  the  lignified  walls  and  the  cork  cells. 
Mount  in  water  and  examine  first  with  low  power.  On  the 
outer  edge  is  a  green  layer — the  cork  cells — scarcely  at  all 
affected  by  the  macerating  fluid.  Next  to  this  is  a  transparent 
layer  of  unstained  parenchyma  cells  more  or  less  injured  by  the 


82  Vegetable  Histology. 

corrosive  fluid — the  middle  bark.  Then  comes  a  narrow  strand 
of  deeply-stained  cells,  which  on  the  ends  of  the  section  may  be 
seen  to  be  fibrous,  and  also  farther  in  if  the  section  be  not  too 
thick,  in  which  case  there  may  be  observed  how  the  fibres  are 
spliced  over  one  another.  Next  to  the  bast  is  another  colorless, 
narrow  layer  composed  of  the  soft  bast  and  cambium.  Adja- 
cent to  this  is  the  broader  strand  of  green  wood  fibres  and 
vessels. 

Now  tap  gently  with  a  teasing  needle  on  the  cover  glass 
directly  over  the  sections.  The  macerating  fluid  has  dissolved 
the  middle  lamellas,  so  that  by  tapping  the  cells  spread  apart 
and  are  easily  studied  individually.  The  flbres  are  very  long, 
the  wood  fibres  being  often  as  much  as  20  or  30  times  as  long 
as  wide  and  the  bast  fibres  even  longer;  the  ends  taper  to  a 
point,  sometimes,  however,  abruptly,  and  at  times  are  forked. 


Fig,  49.— Longitudinal  view  of  flbres  from  stem  of  Geranium  (Bastln), 

The  walls  are  smooth,  and  with  careful  staining  and  good  light 
delicate  slits  are  seen  running  obliquely  across  the  walls.  Fig. 
49  illustrates  both  bast  and  wood  fibres  in  longitudinal  section. 
Sometimes  bast  and  wood  fibres  are  found  which  are  made  up 
of  a  row  of  two  or  three  cells  and  then  they  have  cross-partitions 
in  the  lumen  or  cavity  of  the  fibre. 

Bast  Fibres  of  Cinchona  Bark. 

Bast  fibres  vary  greatly  in  length  and  lignification.  There 
are  some  extremely  long,  flexible,  slightly  lignified  fibres,  for 
example.  Flax,  Hemp,  Mezereum,  on  one  extreme,  and  very 
short,  brittle,  much  lignified  fibres  on  the  other  extreme,  as 
found  in  Cinchona  bark. 

Make  cross  and  longitudinal  radial  sections  of  Cinchona  bark 
(calisaya  or  succirubra)  that  has  been  softened  in  water  or 
alkali. 

Examine  first  a  cross-section  with  low  power.  In  the  inner 
bark  there  are  numerous  yellowish,  solid-looking  quadrangular 
cells,  the  cavities  of  which  have  been  reduced  nearly  to  a  point 
by  the  great  thickening  of  the  cell-walls.  These  are  the  bast 
fibres.  They  occur  singly  or  in  small  radial  groups  of  three  or 
more.  Incidentally,  the  student  should  note  carefully  the 
structure  of  the  whole  section,  as  an  example  of  a  tj^pical  well- 
developed  bark.  On  the  outer  edge  is  a  thick  layer  of  small 
cork  cells,  deeply  colored  with  natural  plant  pigment.  These 
have  a  tendency  to  flake  off  in  places  and  are  divided  here  and 


Fibres. 


83 


there  by  transverse  clefts.  Next  to  the  cork  is  the  middle  bark, 
consisting  of  lighter  colored,  tangentially  elongated  paren- 
chyma cells.  In  it  is  a  tangential  row  of  secretion  channels. 
Next  to  the  middle  bark  is  the  inner  or  bast  bark,  which  is 
divided  into  compartments  by  distinct  radial  lines  of  cells,  two 
or  three  rows  wide,  which  are  the  medullary  rays.  Between 
these  rays  occur  the  bast  fibres  and  a  tangled  network  of 
smaller  thin-walled  cells,  consisting  of  ordinary  parenchyma 
and  sieve  tubes,  the  latter  not  being  easily  made  out. 

Under  high  power  the  excessively  thick- 
ened walls  of  the  bast  fibres  are  made  up  of 
layers  which  are  finely  stratified.  The  walls 
are  also  crossed  by  radiating  fine  canals 
(Fig.  50) .  In  a  section  stained  with  methyl- 
green  solution  the  walls  are  seen  to  be 
strongly  lignified,  but  more  so  in  the  ex- 
terior layers  than  in  those  next  to  the  cavity, 
as  is  shown  by  the  different  depths  of  color. 

Longitudinal  Section. — In  this  the  cork 
and  parenchyma  look  pretty  much  as  in 
cross-section,  but  the  secretion  space,  if  the 
section  contain  one,  appears  as  a  long,  empty 
channel,  and  the  medullary  rays  appear  as 
isolated  plates  of  cells  lying  across  the  sec- 
tion. The  bast  fibres,  which  are  seen  best 
at  the  ends  of  the  section,  are  relatively 
very  short,  being  about  six  times  as  long  as 
wide,  fusiform  or  wedge-shaped  on  the  ends. 
The  cavity  appears  as  a  line  and  the  strati- 
fication is  distinct.  The  fibers  are  also 
marked  by  cross-lines  radiating  from  the 
cavity  (Fig.  50). 

Macerate  a  section  in  Schulze's  solution, 
as  in  the  case  of  the  Geranium  section,  wash, 
mount  and  separate  by  tapping  on  the  cover  glass.  The  bast 
fibres  are  separated  and  may  be  studied  more  easily.  Stain  a 
macerated  section  in  methyl-green,  tap  out  on  a  slide  and  note 
the  effect  of  the  stain. 


Fl, 


g.  50.— Bast  fibres  of 
Cinchona  calisaya  In 
cross  and  longitudi- 
nal section,  showing 
narrow  cavity,  pore 
canals  and  stratifi- 
cation of  walls  (re- 
duced, from  Bastin ) . 


84  Vegetable  Histology. 

CHAPTER    XX. 

Tracheary  Tissue. 

This  includes  tracheids  and  ducts.  When  speaking  of  the 
fibro-vascular  bundles,  it  was  said  that  the  xylem  portion  of 
the  bundle  was  that  which  contains  the  wood  cells  and  tracheids 
or  ducts.  The  peculiarity  of  tracheary  cells  is  that  the  walls 
are  thickened  unevenly,  the  thick  parts  being  on  the  inside  of 
the  walls  and  arranged  in  various  forms,  giving  rise  to  spiral, 
reticulate^  scalariform,  annular,  dotted  ducts,  etc. 

The  cells  of  tracheary  tissue  are  usually  less  thick-walled 
than  wood  fibres  and  of  larger  diameter,  and  mostly  oblique- 
ended  or  blunt.  They  are  more  widely  distributed  than  wood 
fibres,  as  they  are  found  in  all  vascular  plants,  i.  e.,  plants  con- 
taining bundles.  (The  name  fibro-vascular  means  that  there 
are  both  fibres  and  vessels  or  ducts  in  the  bundles.) 

Ducts  and  tracheids  are  alike  in  regard  to  the  markings  of 
the  walls.  The  difference  is  that  a  duct  is  composed  of  a  row 
of  tracheids  in  which  the  separating  partitions  have  been  ab- 
sorbed, leaving  a  duct  or  vessel.  A  tracheid  is  a  single  cell 
complete  on  all  sides.  The  name  tracheary  signifies  the  resem- 
blance between  some  of  the  ducts  and  the  trachea  or  wind-pipe 
of  man. 

Tracheary  Tissue  in  Geranium  Stem. 

Make  longitudinal  radial  sections  and  macerate  in  Schulze'a 
solution,  wash  and  stain  in  methyl-green,  mount  in  water.  The 
following  ducts  may  be  found  in  the  xylem  of  the  bundles, 
especially  if  the  cells  be  separated  by  tapping  upon  the  cover 
glass. 

Reticulate  Ducts. — So  called  because  the  thickenings  in  the 
cell-wall  are  in  the  form  of  a  network,  giving  a  pitted  appear- 
ance to  the  wall.  The  walls  are  somewhat  prismatic  and  the 
pits  occur  on  the  flat  sides.  By  means  of  them  a  lateral  osmotic 
circulation  is  kept  up  with  other  ducts.  Notice  in  some  of  the 
ducts,  on  the  oblique  ends,  large  openings  where  one  cell  com- 
municates with  another.  Reticulate  ducts  are  the  most 
numerous  in  the  Geranium,  as  well  as  most  plants  (Fig.  51). 

Spiral  Ducts. — Widely  distributed  in  plants.  The  thicken- 
ing in  the  wall  takes  place  in  a  perfect  spiral,  the  part  between 
the  turns  being  very  thin  and  almost  invisible  and  cellulose  in 
nature.  There  may  be  two  spirals  in  a  duct,  one  within  the 
other.  In  other  plants  ducts  with  more  than  two  spirals  are 
found.  Sometimes  the  turns  of  the  spirals  are  connected  by 
cross-thickenings,  and  then  the  duct  merges  into  a  reticulate 
one.     Again,  the  thickenings  may  be  spiral  on  one  end  of  the 


Tracheary  Tissue. 


85 


duct,  but  pass  into  separate  circles  on  the  other,  forming  an 
annular  duct.  This  kind  of  duct  is  found  associated  with  the 
spirals  in  the  Geranium.  The  two  are  closely  related.  The 
spirals  and  rings  may  be  close  together  or  wide  apart  and  the 
rings  may  be  at  various  inclinations  to  the  length  of  the  duct. 
The  spiral  ducts  are  smaller  in  diameter  than  the  reticulate 
(Fig.  51). 

SCALARIFORM    DuCTS    IN    FeRNS. 

These  ducts  have  their  thickenings  arranged  like  the  rounds 
of  a  ladder,  hence  the  name  scalariform.  They  occur  in  many 
plants,  but  they  are  seen  to  best  advantage  in  the  ferns,  where 
they  are  beautifully  developed  and  almost  to  the  exclusion  of  all 
other  woody  tissue  in  the  bundles. 

Make  thin  longitudinal  radial  sections  of  the  rhizome  of  the 
official  Male  Fern  or  of  the  Eagle  Fern  (Pteris  aquilina) ,  stain  in 
methyl-green  or  phloroglucin  and  hydrochloric  acid  and  mount 
in  water.  The  ducts  will  be  stained  green  or  red;  they  are 
mostly  prismatic  or  flat-sided  where  they  press  upon  one  an- 
other, and  at  the  edges  where  the  sides  meet  there  is  a  thickened 
ridge.  Crossing  the  flat  faces  are  numerous  parallel  thick 
ridges  separated  by  very  thin  places  looking  like  slits.  The 
faces  have  the  appearance  of  a  ladder.  The  ducts  are  oblique 
or  taper-pointed,  large  in  diameter  and  the  ends  splice  over  one 
another  (Fig.  51). 

Isolate  the  ducts  by  maceration  in  Schulze^s  solution  as  in  the 
previous  cases. 


Fig.  61.— Various  tracheary  ducts.    1  and  2,  spiral ;  3,  combined  annular  and  spiral ; 
4,  scalariform  ;  5,  reticulated. 


Tracheary  Tissue  of  Gymnosperms. 

Tracheids  With  Bordered  Pits.  —  These  peculiar  cells, 
although  occasionally  found  in  other  plants,  are  characteristic 
of  gymnosperms.  In  these  plants  ducts  and  wood  cells  are  rare, 
being  replaced  by  tracheids,  which  constitute  nearly  the  whole 


86 


Vegetable  Histology. 


of  the  wood.  The  tracheids  are  so  peculiar  in  structure  that 
one  may  distinguish  a  gymnosperm  from  other  plants.  A  few 
genera  of  gymnosperms  can  be  recognized  by  the  number  and 
regularity  of  the  markings  on  the  tracheids. 

Soften  a  piece  of  pine  wood  by  soaking  a  long  time  in  dilute 
alkali.  After  washing  thoroughly  in  water,  make  longitudinal 
radial  sections  by  cutting  at  right  an- 
gles to  the  rings  of  growth.  Mount  a 
thin  section  in  water  or  glycerin.  The 
tracheids  are  long,  tapering  fibres,  sim- 
ilar to  wood  fibres,  but  larger.  The  sur- 
faces of  the  cells  are  marked  by  a  row 
of  pits,  each  pit  being  surrounded  by  a 
smaller  circle  inside  of  a  larger  one.  At 
times  the  pits  are  close  together;  at 
other  times  they  are  wide  apart.  Lying 
across  the  fibres  at  intervals  are  rows 
or  plates  of  cells  of  the  medullary  rays. 
Try  the  action  of  phloroglucin  and  hy- 
drochloric acid  on  a  section. 

Now  make  longitudinal  tangential 
sections  by  cutting  parallel  with  the 
rings  of  growth.  In  such  a  section  no 
pits  are  found  on  the  faces  of  the  cells, 
but  they  may  now  be  seen  in  section  on 
the  edge  in  the  cell-wall.  In  other 
words,  the  peculiar  pits  occur  only  on 
the  radial  faces  of  the  cell-walls. 

Find  a  pit  cut  exactly  through  the  middle.  It  forms  a  lens- 
shaped  cavity  in  the  wall  between  two  cells,  opening  into  the 
two  cell  cavities  by  circular  orifices  which  in  flat  view  appear 
as  the  inner  small  circle  of  the  pit  (see  above).  Running 
lengthwise  through  the  pit  and  closing  off  the  cavities  of  the  two 
neighboring  cells  is  the  middle  lamella.  Here  and  there  be- 
tween the  tracheids  occur  rows  of  two  or  three  rounded  cells, 
which  are  not  to  be  mistaken  for  the  pits.  They  are  much 
larger,  being  the  lignified  parenchyma  cells  which  form  the 
so-called  medullary  rays. 


Fig.  52.— Tracheids  of  pine 
allowing  bordered  pits,  with 
medullary  rays  crossing  the 
tracheids   (Sachs). 


Latex  or  Milk  Tubes.  87 

CHAPTEK    XXI. 

Latex  or  Milk  Tubes. 

Many  plants,  when  wounded,  emit  a  milky  fluid  varying  in 
color,  copiousness,  consistency  and  chemical  composition  in 
different  plants.  This  is  called  the  latex,  hence  the  name  lati- 
ciferous  tissue.  This  tissue  differs  considerably  in  different 
plants  and  is  not  confined  to  any  particular  region  or  tissue 
system,  but  it  is  most  common  and  abundant  in  ordinary  par- 
enchyma. 

The  cells  of  milk  tissue  are  of  two  kinds — simple  and  com- 
plex or  branching. 

The  simple  latex  tubes  consist  of  long  cells  of  indefinite 
length  running  lenthwise  of  the  plant  with  only  a  few  branches. 
Each  tube  with  its  few  branches  is  believed  to  be  a  single  cell. 
In  cross-section  they  are  distinguished  from  parenchyma  cells 
by  their  smaller  diameter  and  by  being  filled  with  opaque  and 
densely  granular  substances.  The  cell-walls  are  cellulose.  The 
complex  tissue  consists  of  greatly  branching  tubes,  the 
branches  uniting  cross-wise  and  forming  a  complex  network  in 
the  plant. 

Latex  is  of  the  nature  of  a  waste  or  excretory  product, 
although  it  contains  albumin  and  carbohydrates.  It  contains 
resins  and  gums  in  solution  and  oily  matters,  often  alkaloids 
and  organic  acids.  It  coagulates  and  forms  a  sticky  mass  upon 
exposure  to  the  air.  India  rubber  is  an  example  of  such  dried 
latex.  Latex  varies  in  color  in  different  plants;  it  may  be 
white,  yellow,  orange,  etc.     In  Bloodroot  it  is  reddish. 

Stems  of  plants  in  which  latex  tissue  is  to  be  studied  should 
be  cut  into  pieces  and  immediately  put  into  strong  alcohol, 
which  coagulates  the  latex  and  prevents  it  from  running  out  of 
the  tubes. 

Simple  Latex  Tubes. 

These  may  be  studied  conveniently  in  Euphorbia  and  Milk- 
weed plants.  Make  cross-sections  of  the  stem  of  one  of  the 
Milkweeds,  stain  in  methyl-green  and  mount  in  water.  The 
milk  tubes  occur  in  the  pith  and  bark  and  are  distinguished 
from  the  neighboring  parenchyma  cells  by  their  smaller  size 
and  densely  granular,  more  deeply-stained  contents. 

Examine  a  longitudinal  section  stained  in  methyl-green. 
Lying  among  the  parenchyma  cells  will  be  found  long  tubes 
filled  with  dense,  granular  matter,  wavy  and  with  only  an  occa- 
sional branch.  The  branches,  when  present,  do  not  unite  with 
those  of  a  neighboring  tube.  Each  tube,  however  long,  may  be 
looked  upon  as  a  single  cell. 


88  Vegetable  Histology. 

Apply  chlor-zinc-iodine.  The  cell-walls  stain  blue  and  the 
contents  a  brown,  showing  presence  of  albuminous  matter. 
Alcannin  or  cyanin  solutions  would  show  the  presence  of  oily 
or  resinous  matters.  These  are  held  in  suspension  by  emulsion 
and  give  the  latex  the  milky  appearance. 

Complex  or  Branching  Latex  Tubes. 


for  example,  Dandelion, 
Chicory  roots,  stain 


These  occur  in  a  number  of  plants 
Chicory,  Celandine,  Poppy,  etc. 

Make  cross-sections  of  Dandelion  and 
with   hsematoxylin   so- 
lution and  wash  thor- 
oughly in  water. 

The  milk  cells  are 
arranged  in  small 
groups,  and  these  form, 
in  the  Dandelion,  con- 
centric circles  through- 
out the  whole  bark, 
which,  to  the  naked 
eye  and  under  low 
power,  seem  complete. 
Under  high  power  the 
circles  are  interrupt- 
ed here  and  there.  In 
Chicory  the  milk  tubes 
are  arranged  in  radi- 
ating lines  through  the 
bark,  and  by  means  of 
the  milk  tubes  alone 
Dandelion  and  Chicory 
can  readily  be  distinguished  under  the  microscope.  The  milk 
cells  stain  more  deeply  than  the  surrounding  parenchyma  cells 
and  thus  stand  out  conspicuously. 

Study  longitudinal  radial  section  of  Dandelion  and  longitu- 
dinal tangential  section  of  Chicory. 

The  milk  ducts  form  a  tangled  network  in  the  elongated 
parenchyma  cells  of  the  bark  of  the  root.  There  are  numerous 
cross-branches  connecting  the  tubes. 


Fig,  53.— Longitudinal  radial  section  of  Chicory 
root,  siiowing  branching  latex  tubes,  sen 
(Moeller). 


FiBRo- Vascular  Bundles  and  Types  op  Stems.         89 

CHAPTER   XXII. 

FiBRo- Vascular  Bundles  and  Types  of  Stems. 

The  nature  of  a  fibro-vascular  bundle  was  considered  in  the 
lessons  on  wood  and  bast  fibres  and  tracheary  tissue,  but  the 
various  types  of  bundles  were  not  discussed.  These  will  be 
considered  now. 

According  to  the  relative  arrangement  of  the  xylem  and 
phloem  masses  three  kinds  of  fibro-vascular  bundles  are  dis- 
tinguished, namely,  collateral^  concentric  and  radial. 

The  collateral  type  is  characterized  by  having  the  xylem  and 
phloem  masses  lying  side  by  side  with  the  xylem  facing  to- 
wards the  pith  or  center  of  the  stem  and  the  phloem  towards 
the  exterior.  In  the  veins  of  leaves  the  xylem  faces  the  upper 
or  ventral  surface,  the  phloem  the  lower  or  dorsal  surface.  Col- 
lateral bundles  are  characteristic  of  the  stems  and  leaves  of 
nearly  all  flowering  plants.  They  seldom  occur  in  roots.  There 
are  two  varieites  of  the  collateral  type : 

The  ordinary  bundle,  containing  one  phloem  and  one  xylem 
mass,  and  the  hicollateral  bundle,  in  which  there  is  one  xylem 
mass  between  two  phloem  masses  or  vice  versa.  The  second 
variety  is  found  only  in  the  stems  of  gourd  plants  (Cucurbi- 
tacese)  and  a  few  others.  Some  collateral  bundles  continue  to 
increase  in  thickness  during  the  life  of  the  plant,  and  the  grow- 
ing layer  is  located  at  the  junction  of  the  xylem  and  phloem, 
forming  a  cambium  or  meristem  zone  of  the  bundle.  Such  bun- 
dles are  called  open  bundles,  while  those  which  have  no  cam- 
bium zone,  and  thus  soon  cease  to  grow,  are  called  closed 
bundles. 

The  open  bundles  are  characteristic  of  the  stems  of  woody 
dicotyls.  An  illustration  of  this  kind  of  bundle  has  been  seen 
in  the  Geranium  stem.  The  stems  of  most  monocotyls  contain 
the  closed  collateral  bundles. 

Concentric  Bundles. — These  have  a  central  xylem  mass  sur- 
rounded by  a  phloem  mass  or  vice  versa.  There  is  no  cambium 
zone  in  this  type.  The  bundle  with  xylem  central  is  character- 
istic of  nearly  all  ferns  and  club  mosses.  The  one  with  phloem 
central  occurs  only  in  stems  and  leaves  of  some  monocotyls. 

Radial  Bundles. — In  these  the  xylem  tissues  are  arranged  in 
radial  masses  and  are  separated  from  one  another  by  the  phloem 
masses,  together  with  some  parenchyma  cells.  Such  bundles 
are  characteristic  of  the  roots  of  all  phanerogams  and  pterido- 
phytes  and  stems  of  Lycopodiaceae. 


90 


Vegetable  Histology. 


The  following  scheme  shows  the  types  of  bundles  and  their 
distribution : 

a.  Open  Bundles, 
— Stems  and  leaves 
of  woody  dicotyls. 
(Cambium  zone 
present.) 

6.  Closed  Bun- 
dles. —  Most  mono- 
cotyls,  the  ferns 
mentioned  and 
stems  of  Equiseta- 
cese.  (No  cambium 
zone  present.) 


Collateral  Bun- 
dles. —  Stems  and 
leaves  of  nearly  all 
flowering  plants,  a 
few  ferns,  as  the  gen- 
era Ophioglossum 
and  Osmunda  and 
stems  of  Equiseta- 
ceae. 


I.  Ordinary  Bun- 
dles. —  Having  one 
phloem  and  one 
xylem  mass. 


II.  Bicollateral 
Bundles.  —  Chiefly 
stems  of  Cucurbi- 
tacese    (gourd   fam- 

ly)- 

I.  Bundles  with  xylem  central. 
Stems  and  leaves  of  nearly  all  ferns 
and  club  mosses. 

II.  Bundles  with  phloem  central. 
Stems  and  leaves  of  some  mono- 
cotyls. 

Radial  Bundles. — Roots  of  all  phanerogams  and  pterido- 
phytes  and  stems  of  Lycopodiacese. 


Concentric  Bundles 


Collateral  Bundles. 

Closed  Bundles. — Harden  a  stout  piece  of  the  stem  of  Spider- 
wort  ( Tradescantia  Virginica)  in  alcohol  and  make  thin  cross- 
sections.     (This  is  a  monocotyl).     Stain  with  phloroglucin. 

The  greater  portion  of  the  section  is  made  up  of  large  ordinary 
parenchyma  cells  containing  starch.  Scattered  among  these 
cells  over  the  entire  section  are  numerous  rounded  areas  of 
smaller  cells  containing  no  starch.  These  are  the  closed  col- 
lateral bundles.  In  the  xylem,  which  faces  towards  the  center 
of  the  section,  will  be  found  from  three  to  five  thick-walled 
ducts,  which  in  many  of  the  bundles  are  arranged  in  the  form 
of  a  V.  In  some  of  the  bundles  the  xylem  completely  surrounds 
the  phloem.  On  the  side  of  the  xylem  towards  the  center  of  the 
section  there  is  usually  found  a  large  intercellular  space.  This 
is  often  found  in  closed  collateral  bundles. 

The  phloem  is  composed  of  small  cells,  which  are  mostly 
sieve  cells,  accompanied  by  some  parenchyma  cells.  There  is 
no  growing  or  cambium  zone  between  the  phloem  and  xylem. 


MoNocoTYL  Stem.  91 

The  bundles  are  enclosed  by  a  single  row  of  cells  smaller  in 
diameter  than  the  other  parenchyma  cells  of  the  section  and 
containing  little  or  no  starch.  This  is  the  endodermis  or  hundle 
sheath.  It  is  poorly  developed  in  this  type  of  bundle  and  is 
often  not  present  at  all.  It  will  be  met  with  later  in  perfectly 
developed  form  in  the  concentric  and  radial  bundles. 

Note  the  arrangement  of  the  whole  section  of  this  stem.  It 
represents  the  type  of  structure  of  all  monocotyl  stems.  The 
bundles  are  scattered  without  any  definite  order  over  the  whole 
cross-section,  though  they  are  more  numerous  in  the  outer  part 
than  towards  the  center  of  the  section.  There  is  no  true  bark 
as  exists  in  stems  of  dicotyls. 

The  student  should  be  careful  to  differentiate  between  type 
of  structure  and  details  of  structure.  Other  monocotyl  plants 
have  the  same  plan  of  arrangement  as  in  the  one  just  studied, 
but  perhaps  none  looks  exactly  the  same.  The  case  is  very 
much  like  that  of  human  beings,  all  of  whom  are  constructed 
on  the  same  plan,  but  very  rarely  do  two  tally  so  closely  that 
they  cannot  be  distinguished. 

To  emphasize  the  above  point,  study  a  cross-section  of  the 
young  stem  of  Greenbrier  stained  with  phloroglucin.  If  the 
stem  be  too  hard  to  cut  easily,  soften  it  by  soaking  in  dilute 
alkali  (1  or  2  per  cent.)  sufficiently  long.  Greenbrier  is  a 
monocotyl  plant  of  the  harder  or  more  woody  kind,  while 
Spiderwort  represents  the  soft,  herbaceous  plants.  In  the  sec- 
tion will  be  seen  numerous  bundles  of  the  closed  collateral 
variety  containing,  besides  vessels,  both  wood  and  bast  fibres. 
The  bundles  are  scattered  almost  over  the  whole  area  of  the 
section  and  are  imbedded  in  a  groundwork  of  parenchyma  cells, 
which,  however,  have  their  wall  considerably  thickened,  thus 
differing  from  the  parenchyma  of  Spiderwort.  The  area  con- 
taining the  bundles  is  sharply  divided  from  a  narrow,  outer 
rim  of  the  section,  which  resembles  in  a  way  the  bark  of  a 
dicotyl  plant.  Most  monocotyl  stems  have  such  a  more  or  less 
distinct  dividing  line.  In  some  it  is  a  single  chain  of  cells,  in 
others  it  is  strongly  developed;  it  is  known  as  the  cylinder 
sheath.  In  the  Greenbrier  it  consists  of  numerous  hard  fibrous 
cells,  which  result  from  the  crowding  of  incomplete  bundles  in 
this  region. 

Note  the  two  very  large  ducts  and  several  smaller  ones  in  the 
xylem  or  wood  portion  of  the  bundles.  The  xylem  portion  is 
directed  towards  the  center  of  the  section.  In  the  outer  portion 
next  to  the  two  large  ducts  is  an  area  of  softer  smaller  cells, 
the  phloem,  made  up  of  sieve  tubes  and  some  parenchyma  cells. 
The  whole  bundle  is  enclosed  by  fibres  which,  on  the  inner  edge, 
are  called  wood  fibres,  while  on  the  outer  edge  they  are  called 
bast  fibres. 


92 


Vegetable  Histology. 


Fig.  54. — Cross-section  of  stem  of  Greenbrier,  a,  cortex ;  b,  cylinder  sheath,  made 
up  of  incomplete  bundles ;  c,  a  bundle ;  d,  parenchyma  cells  of  ground  tissue 
(Bastin). 


Pig.  55.— Section  of  a  bundle  of  Greenbrier,  magnified,    a,  large  duct  in  xylem ;    b, 
smaller  duct;    c,  phloem  mass;    d,  parenchyma  cell    (Bastin). 

Open  Bundles. — These  are  found  in  stems  of  dicotyledonous 
plants.  In  herbs  they  are  more  or  less  isolated,  and  the  xylem 
portions  do  not  form  a  solid  continuous  woody  cylinder,  as  in 
shrubs  and  trees.  The  cross-section  of  the  Geranium  stem  fur- 
nished an  example  of  open  collateral  bundles. 

To  study  the  bundles  in  a  woody  type  of  stem,  make  cross- 
sections  of  the  stem  of  Bittersweet  and  stain  with  phloroglucin. 
Surrounding  a  central  area  of  pith  cells  will  be  found  a  thick 
ring  of  cells  that  stain  red,  composed  mainly  of  thick-walled 
wood  fibres,  which  are  interspersed  with  cells  of  much  larger 
diameter,  the  ducts.     This  solid  ring  of  cells  is  composed  of  the 


DicoTYL  Stem. 


xylem  portions  of  numerous  bundles  which  have  grown  toffether 
and  the  only  evidence  left  of  their  having  once  been  separated  is  a 
number  of  radial  rows  of  elongated  cells  running  through  the 
ring  from  the  pith  to  the  outer  edge.  These  rows  of  cells  are  the 
medullary  rays  The  cells  were  once  soft  parenchyma  cells 
between  the  bundles,  but  have  subsequently  become  lignified 

At  the  outer  edge  of  the  ring  of  wood  cells  is  a  narrow  zone 
of  small  cells,  thm-walled,  unstained,  rectangular  in  shape  and 
more  or  less  in  radial  rows.  These  form  the  cambium  zone  or 
growing  layer. 

Outside  the  cambium  is  a  zone  of  unstained  cells  composed 
of  sieve  tissue  and  parenchyma  cells.    This  zone  is  bordered 


wood 


Fig.   56.— Segment  of  cross-section  of  stem  of  Bittersweet,     cort,   parenchyma   of 
middle  bark;    scl.f.,  bast  fibres;    m.r.,  medullary  ray  (Greenish). 

exteriorly  by  a  broken  circle  of  small  thick-walled  bast  fibres. 
The  area  between  the  cambium  and  bast  fibres,  including  the 
latter,  is  composed  of  the  phloem  portions  of  the  bundles.  The 
medullary  rays  are  continued  through  the  phloem  areas.  This 
region  forms  the  so-called  inner  or  liber  bark,  spoken  of  in  the 
lesson  on  bast  and  wood  fibres.  Next  to  the  inner  bark  is  the 
middle  bark,  composed  of  large  ordinary  parenchyma  cells. 
Beyond  this  is  the  outer  bark,  composed  of  a  layer  of  cork  cells. 
Only  dicotyls  have  a  true  bark,  and  the  structure  of  such  a  bark 
is  seen  in  this  section.  When  the  bark  is  peeled  off  the  rupture 
takes  place  at  the  cambium  zone,  which  is  soft  and  easily  torn. 


94 


Vegetable  Histology. 


In  perennial  dicotyl  stems  the  cambium  zone  forms  yearly  a 
new  layer  of  woody  tissue  on  its  inner  edge,  thus  increasing 
the  diameter  of  the  plant.  Moreover,  the  beginning  and  end  of 
the  year's  growth  usually  differ  in  appearance,  and  thus  the 
"rings  of  growth"  are  distinguished,  and  by  their  number  the 
age  of  the  plant  also.  Monocotyl  stems  have  no  cambium  in 
the  bundles  after  they  are  mature,  hence  such  stems  soon  cease 
to  increase  in  diameter.     They  are,  as  a  rule,  slender  stems. 

Study  the  whole  cross-section  as  a  type  of  the  more  woody 
dicotyledonous  stems  and  contrast  it  with  the  stem  of  the 
Spiderwort  studied  above.  It  is  the  definite  arrangement  of  the 
bundles  in  a  single  circle,  with  xylem  ends  centrally  and  phloem 
ends  exteriorly,  that  gives  rise  to  a  true  bark  and  a  central 
woody  cylinder  in  dicotyls. 

Make  cross-sections  of  stem  of  Lizard's  Tail  as  a  type  of  herba- 
ceous dicotyledonous  plants. 

Stain  with  phloroglucin.  The  bundles  are  arranged  in  a 
circle,  but  are  not  grown  together.  They  are  separated  by  soft 
parenchyma  tissue,  which  forms 
the  greater  portion  of  the  sec- 
tion. There  is  a  cambium  zone 
between  phloem  and  xylem.  In 
the  xylem,  next  the  cambium, 
are  some  large  ducts  with  a  few 
parenchyma  cells  mixed  in. 
Next  to  these  are  some  ducts 
of  smaller  diameter  and  then  a 
semicircular  zone  of  thick-wall- 
ed wood  fibres  marking  the 
limits  of  the  xylem  of  the  bun- 
dle. In  the  phloem,  next  to  the 
cambium,  is  a  mass  of  soft  tis- 
sue composed  of  sieve  cells,  par- 
enchyma cells  and  a  few  secre- 
tion cells.  The  phloem  is 
bounded  by  a  layer  of  thick- 
walled  bast  fibres  with  pore- 
canals.  This  layer  passes 
around  to  meet  the  layer  of 
wood  fibres  of  the  xylem,  forming  thus  a  sort  of  sheath  to  the 
bundle. 

Note  that  the  arrangement  of  the  tissues  in  this  stem  is  the 
same  as  that  in  the  Bittersweet,  the  difference  being  one  of  de- 
gree rather  than  kind.  There  is  far  less  wood  in  the  section. 
The  type  of  bundles  and  the  arrangement  in  a  single  circle  is 
the  same. 

Sketch  the  whole  cross-section  of  Bittersweet  and  Lizard's 
Tail  and  make  a  more  detailed  drawing  of  a  segment  of  each 
section. 


Pig.  57.— Cross-section  of  a  bundle  of 
Lizard's  Tail,  a,  parenchyma  ;  b,  air 
space  ;  c.  larg&  duct ;  d,  bast  fibres  ; 
h,  wood  fibres  ;  e,  soft  bast ;  f ,  cam- 
bium ;    g,  smaller  duct  (Bastin). 


Fern  Stem. 


95 


Bl-COLLATERAL   BUNDLES. 

Make  cross-sections  of  the  hardened  stem  of  Pumpkin,  Squash 
or  Watermelon. 

The  bundles  consist  of  a  xylem  mass  betwen  an  outer  and  an 
inner  phloem  mass.  In  the  xylem  are  some  very  large  ducts, 
looking  like  large  holes,  with  some  smaller  ones  in  the  inner 
portion.  There  is  a  large  quantity  of  small-celled  parenchyma 
tissue.  The  phloem  masses  consist  of  large  sieve  cells  and 
accompanying  parenchyma  cells.  There  are  no  bast  fibres 
present  nor  any  wood  fibres  in  the  xylem.  There  are  two  layers 
of  cambium  cells,  one  between  the  xylem  and  outer  phloem 
mass,  the  other  between  the  xylem  and  inner  phloem  mass.  The 
rest  of  the  section  is  made  up  of  very  large-celled  parenchyma 
tissue. 

Concentric  Bundles. 

I.    Variety  with  phloem  surrounding  xylem. 

Make  cross-sections  of  the  rhizome  of  the  Eagle  Fern  (Pteris 
aquilina)  and  stain  with  phloroglucin. 

On  the  exterior  is  the  epidermis,  brown  in  color,  then  several 
layers  of  thick-walled  fibrous  cells,  also  brown,  known  as  the 
hypoderma.  Interior  to  this  is  a  zone  of  large  ordinary  par- 
enchyma cells  with  starch  grains.  Then  comes  a  circle  of  bun- 
dles, separated  from  one  another  by  parenchyma.  The  bundles 
may  be  round  or  elongated,  but  never  radially.  Within  this 
circle  of  bundles  are  two  elongated  masses  of  thick-walled, 
fibrous  cells,  dark  colored.  Lying  between  these  are  two  bun- 
dles, elongated  and  larger  than  those  of  the  circle.  The  spaces 
between  the  objects  just  described  are  filled  up  with  paren- 
chyma cells. 

In  the  center  of  the  bundles  is  the  xylem,  composed  of  large 
scalariform  ducts  stained  red  and  a  few  small  parenchyma  cells. 
Surrounding  the  xylem 
is  the  phloem,  unstain- 
ed, consisting  of  a  layer 
of  small  parenchyma 
cells  with  fine  starch 
grains  immediately  next 
to  the  xylem;  then  a 
layer  of  sieve  cells  and 
their  companion  cells, 
finally  another  layer  of 
small  starch  -  bearing 
parenchyma  cells.  The 
bundle  is  sharply  divid- 
ed off  from  the  sur- 
rounding parenchyma 
by  a  well-developed  ring 
of  elongated  prismatic 
cells,  known  as  the  en- 
dodermis  or  bundle  sheath. 


Fig.    58.-Slightly 
Eagle  Pern 


magnified   section   of   stem   of 


_  a,  hypoderma ;    b,  small  group  of 

dark  fibres  in  cortex  ;  c,  one  of  bundles  in  a  cir- 
cle ;  d.  one  of  the  two  bundles  in  the  center ;  e, 
brown  mass  of  fibrous  cells  (Bastin). 


96 


Vegetable  Histology. 


There  is  no  cambium  in  the  bundles,  hence  fern  stems,  just  as 
those  of  monocotyl  plants,  do  not  increase  in  diameter  from 
year  to  year.     They  remain  rather  slender. 

Note  the  plan  of  this  stem.  It  is  the  type  on  which  all  ferns 
are  built.  Note  also  that  it  differs  from  the  monocotyl  and 
dicotyl  types  of  stem.  The  two  large  masses  of  dark  fibrous 
cells  in  the  central  portion  of  the  section  are  not  present  in  all 
ferns. 

II.     Variety  with  xylem  surrounding  phloem. 

Make  cross-sections  of  stems  of  False  Solomon's  Seal  or 
Sweet  Flag,  both  monocotyl  plauts,  and  stain  with  phloroglucin. 

The  structure  of  the  bundles  is  just  the  reverse  of  that  of  the 
bundles  of  the  fern,  in  that  the  xylem  cells  are  on  the  outside  and 
the  phloem  is  central,  but 
there  is  no  endodermis  pres- 
ent. In  a  few  bundles  the 
xylem  ring  is  incomplete 
and  the  phloem  is  continu- 
ous with  the  parenchyma 
outside  the  bundle.  The 
concentric  bundles,  with 
phloem  central,  may  be  re- 
garded as  closed  collateral 
bundles  in  which  the  xylem 
has  completely  grown  around 
the  phloem  mass. 

Both  these  plants  have  the 
arrangement  of  the  parts 
characteristic  of  mono- 
cotyl stems.  In  both  there 
is  a  cylinder  sheath  divid- 
ing an  outer  border  or  cortex  from  the  broad  central  area  in 
which  the  bundles  are  scattered.  The  parenchyma  tissue  of 
Sweet  Flag  (Calamus)  has  been  studied  earlier  (Chapter 
XVIII).  The  parenchyma  of  False  Solomon's  Seal  is  large, 
thin-walled,  closely  packed.     Neither  section  contains  cork  cells. 

Radial  Bundles. 

These  vary  considerably  among  themselves  in  regard  to  the 
number  of  xylem  rays  and  their  length,  the  amount  of  lignifica- 
tion  of  the  cells,  the  structure  of  the  pericambium  layer  and  of 
the  endodermis.  The  number  of  rays  varies  from  two  to  forty 
or  fifty  in  different  roots.  The  number  of  rays  is  indicated  by 
the  word  arch  with  a  numeral  prefixed.  Thus  a  bundle  with 
two  xylem  rays  is  called  a  diarch  bundle,  one  with  three  a 
triarch  bundle,  etc.  As  a  rule,  dicotyl  and  gymnosperm  roots 
have  fewer  rays  and  a  thinner-walled  endodermis  than  roots  of 
monocotyls. 


Fig.  59.— Cross-section  of  a  concentric  bun- 
dle of  the  rhizome  of  Sweet  Flag,  with 
central  phloem  surrounded  by  the  xylem 
vessels  (Tschirch). 


Roots.  97 

Root  of  a  Dicotyl  Plant.— Make  cross-sections  of  the  root 
of  the  May  Apple  (Podophyllum  peltatum)  and  stain  in  phloro- 
glucin.  In  the  center  of  the  section  will  be  found  the  bundle, 
which  in  this  plant  is  usually  pentarch,  i.  e.,  has  five  xylem  rays. 
The  rays  are  wedge-shaped,  with  the  broad  ends  towards  the 
center.  At  the  outer  narrow  ends  of  the  rays  the  ducts  are 
smaller  in  diameter,  but  are  much  larger  at  the  broad  ends  and 
mostly  scalariform.  The  central  portion  of  the  bundle  is  made 
up  of  parenchyma  cells,  among  which  may  be  a  few  scattered 
ducts. 

The  phloem  masses  lie  between  the  xylem  rays,  towards  their 
outer  ends,  and  are  separated  from  them  by  several  layers  of 
parenchyma  cells.  The  cells  of  the  phloem  have  glistening 
walls  and  may  be  told  by  these.  The  bundle  is  enclosed  by  an 
endodermis  of  elongated  cells,  which  are  thin-walled,  as  is  usual 
in  dicotyl  plants.  Immediately  next  to  the  endodermis  are  two 
layers  of  cells,  larger  in  diameter  than  the  cells  of  the  endo- 
dermis or  of  the  phloem  and  containing  some  fine-grained  starch. 
These  cells  are  known  as  the  pericamMum  or  phloem-sheath. 
The  cells  have  the  power  of  multiplication  and  root  branches 
have  their  origin  from  them,  opposite  the  xylem  rays. 

The  area  outside  of  the  bundle  is  filled  with  parenchyma  cells 
densely  filled  with  starch  grains. 

Root  op  a  Monocotyl  Plant. — Make  sections  of  the  root  of 
Yellow  Lady's  Slipper  (Cypripedium  pubescens),  a  monocotyl 
plant,  and  stain  with  phloroglucin.  The  xylem  rays  are  about 
eight  in  number,  longer  and  better  developed  than  in  the  pre- 
vious case.  They  meet  at  the  center  of  the  section,  where  there 
are  numerous  large  ducts  and  smaller  thick- walled  cells.  The 
ends  of  the  rays  are  surrounded  by  thick-walled  narrow  cells 
which  reach  out  to  the  endodermis,  interrupting  the  pericam- 
bium  layer  in  places.  The  phloem  masses  lie  between  the  rays ; 
the  walls  of  the  cells  are  thin  and  glistening.  The  endodermis 
is  peculiar  in  that  the  cells  opposite  the  phloem  masses  have 
their  radial  and  inner  walls  much  thickened,  giving  to  these 
parts  the  appearance  of  a  crescent,  while  the  outer  walls  remain 
thin.  The  other  cells  of  the  endodermis  opposite  the  xylem  rays 
are  thin-walled.  This  is  a  peculiarity  of  the  endodermis  of 
monocotyl  roots.     Make  a  drawing  of  the  bundle. 

Compare  with  the  section  of  Cypripedium,  one  from  the  root 
of  the  corn  plant,  which  is  also  a  monocotyl.  There  are  about 
fifteen  xylem  rays,  somewhat  like  those  of  podophyllum  in  ap- 
pearance. They  do  not  reach  to  the  center  of  the  bundle.  This 
is  filled  up  with  parenchyma  cells,  in  which  there  is  a  circle  of 
five  very  large  vessels. 

The  roots  of  monocotyls  undergo  very  little  change  as  they 
grow  older,  but  while  the  young  roots  of  dicotyls  present  the 
appearance  described  under  the  root  of  Podophyllum,  the  older 


98 


Vegetable  Histology 


ones  undergo  radical  changes  and  assume  the  structure  of 
dicotyl  stems.  In  fact,  the  section  of  old  dicotyl  roots  looks  so 
much  like  that  of  a  stem  that  it  is  often  difficult  to  distinguish 
it  from  a  stem  section.  These  changes  can  easily  be  followed 
by  making  sections  of  a  root  at  various  distances  behind  the 
growing  point. 


Fig.  60.— Cross-section  of  root  of  May  Apple,  showing  radial  bundle,  a,  endodermis ; 
b,  pericambium  ;  c,  xylem  ray  ;  d,  phloem  ;  p,  parenchyma  of  cortex  surround- 
ing the  bundle  (reduced,  from  Bastm). 


CHAPTER    XXIII. 


Leaves. 


The  leaf  consists  of:  (1)  the  fibro-vascular  system  or  frame- 
work of  veins;  (2)  the  parenchyma  or  filling;  (3)  the  epidermis, 
which  covers  the  whole  leaf.  The  parenchyma  or  mesophyll  of 
the  leaf  is  arranged  differently  in  different  leaves,  giving  rise 
to  two  types  of  leaves,  namely,  hifacial  and  centric. 

Bifacial  Leaf. — These  are  always  flat  leaves,  and  in  section 
present  a  distinct  uj^per  and  lower  surface,  which  are  quite  dif- 
ferent in  structure.  The  parenchyma  cells  next  the  upper  sur- 
face are  compactly  arranged  and  elongated  perpendicular  to  the 
surface.  Such  cells  are  known  as  palisade  parenchyma.  They 
contain  numerous  chlorophyll  bodies,  which  give  to  the  upper 
surface  of  such  leaves  the  deeper  green  color,  as  compared  with 
the  lower  surface. 

The  parenchyma  next  to  the  lower  surface  is  loosely  arranged 


Leaves. 


99 


and  scarcely  elongated  at  all,  and  is  known  as  spongy  paren- 
chyma. 

Most  any  flattened  leaf  will  serve  for  the  study  of  the  bifacial 
type.  An  excellent  leaf  is  that  of  the  Rubber  Tree  (Ficus 
elastica),  because  of  its  toughness  and  thickness.  Leaves 
bleached  in  alcohol  will  be  better,  as  the  sections  will  be  more 
transparent. 

Make  sections  perpendicular  to  the  lateral  veins  of  the  leaf 
and  mount  in  water  or  glycerin.  Sections  of  the  fresh  leaf  may 
be  cleared  in  carbolic  acid  or  chloral-hydrate. 

The  epidermis  on  both  surfaces  is  composed  of  three  layers  of 
cells.  This  is  not  common  to  all  leaves,  but  is  usually  found  in 
tough  evergreen  leaves. 
The  triple  layer  affords 
greater  protection.  Here 
and  there  along  the  upper 
surface,  and  sometimes  on 
the  lower  also,  occur  very 
large  cells  with  a  mass 
hanging  from  a  stalk  at- 
tached to  the  cell-wall,  like 
a  bunch  of  grapes,  in  the 
cavity  of  the  cell.  The 
hanging  masses  are  called 
cystoUths.  They  are  not  of 
common  occurrence. 

Next  to  the  upper  epider- 
mis are  two  layers  of  elon- 
gated cells,  the  cells  of  the 
outer  layer  being  much 
longer  than  those  of  the 
inner  layer,  and  all  are  fill- 
ed with  chlorophyll  gran- 
ules. These  are  the  pali- 
sade cells. 

The  rest  of  the  space  below  the  palisade  cells  is  filled  in  with 
spongy  parenchyma.  The  cells  are  not  elongated  and  contain 
much  less  chlorophyll  than  the  palisade  cells.  The  cells  next 
the  lower  epidermis  are  somewhat  compactly  arranged. 

In  the  lower  epidermis  will  be  found  stomata  or  breathing 
pores.  Some  of  the  pores  will  be  found  cut  through  the  middle, 
giving  a  clear  view  of  the  guard  cells.  Each  pore  is  seen  to  lead 
into  a  large  air  space  in  the  leaf.   (See  Fig.  61.) 

The  cystoliths  consist  of  a  groundwork  of  cellulose  infiltrated 
with  calcium  carbonate.  On  adding  a  drop  of  acetic  acid  to  a 
section  the  carbonate  will  dissolve  with  effervescence,  leaving 
the  cellulose  mass,  which  stains  blue  with  chlor-zinc-iodine. 

At  intervals  along  the  section  will  be  found  the  collateral 


Fig.  61,— Cross-section  of  portion  of  leaf  of 
Rubber  Tree,  a,  e,  three-layered  upper  and 
lower  epidermis ;  c,  palisade  cells ;  d, 
spongy  cells;  h,  xylem ;  i,  soft  bast;  k, 
bast  fibres,  of  a  small  vein  ;  f,  stoma ;  g, 
air  space  (reduced,  from  Bastin). 


100  Vegetable  Histology. 

bundles  of  the  veins  with  the  xylem  always  towards  the  upper 
side  and  the  phloem  towards  the  lower  side  of  the  leaf.  The 
upper  and  lower  faces  of  a  leaf  can  always  be  told  by  noting  the 
position  of  the  xylem  and  phloem  of  the  bundles  of  the  leaf. 

Centric  Leaf. — This  type  of  leaf  is  symmetrical,  i.  e.,  the 
structure  on  one  side  is  the  same  as  on  any  other  side.  Palisade 
cells  are  not  present.  Centric  leaves  are  terete,  acicular  or 
succulent,  and,  occasionally,  flattened  leaves  belong  to  the  type. 

Most  any  pine  needle  will  illustrate  the  type;  also  leaves  of 
Lady's  Slipper,  Sweet  Flag,  Hyacinth,  Daffodil. 

Make  cross-sections  of  a  pine  needle  by  holding  it  between 
pith  and  cutting  through  the  latter.  If  necessary,  clear  them  in 
carbolic  acid,  chloral  hydrate  or  Labarraque's  solution. 

The  leaf  is  flat  on  one  side,  which  is  the  upper  or  ventral,  and 
nearly  semi-circular  on  the  other.  The  epidermis  is  a  single 
layer  of  thick-walled  cells  which  possesses  stomata  on  all  sides 
of  the  leaf. 

Next  to  the  epidermis  are  two  or  three  layers  of  thickened 
fibrous  cells,  and  next  to  these  comes  the  parenchyma  of  the  leaf, 
consisting  of  thin-walled  cells,  whose  walls  have  been  infolded, 
forming  a  variety  of  parenchyma  known  as  folded.  The  cells 
contain  chlorophyll  bodies.  Arranged  at  nearly  equal  intervals 
in  the  parenchyma  are  about  five  secretion  reservoirs,  in  which 
the  circle  of  secreting  cells  is  enclosed  by  one  of  thick-walled 
cells. 

A  bundle  sheath  separates  the  central  portion  from  the  rest 
of  the  section,  next  to  which  are  parenchyma  cells  surrounding 
two  collateral  bundles  in  the  center  of  the  section. 

Apply  phloroglucin  and  hydrochloric  acid  and  note  result; 
also  iodine  solution. 


Fig.  62.— Cross-section  of  a  pine  needle  (centric  leaf),  a,  bast  fibres  of  bundle;  b, 
stoma ;  c,  epidermis ;  d,  secretion  channel ;  e,  air  space ;  f,  folded  paren- 
chyma ;  g,  bundle  sheath ;  h,  parenchyma  ;  i,  phloem,  or  soft  bast  of  bundle ; 
k,  xylem  of  bundle  (reduced,  from  Bastin). 


Reagents.  101 

APPENDIX. 
A. — Various  Reagents  Used  in  the  Study  of  Plant  Tissues. 

PERMANENT   STAINS. 

Kleinenberg's  hematoxylin. — Saturate  some  70  per  cent, 
alcohol  with  calcium  chloride,  let  the  mixture  stand  12  to  24  hours 
over  powdered  alum,  shaking  occasionally ;  add  8  parts  of  70  per 
cent,  alcohol,  filter  and  then  add  a  saturated  solution  of  haema- 
toxylin  in  absolute  alcohol  until  a  purple-blue  color  is  pro- 
duced. Let  stand  in  a  corked  bottle  in  sunlight  for  a  month; 
it  is  then  ready  for  use.  The  liquid  is  to  be  diluted  as  required 
with  dilute  alum  solution.  Over-stained  sections  are  brought 
back  to  proper  degree  of  staining  by  washing  in  acidified  70  per 
cent,  alcohol  (4  to  6  drops  hydrochloric  acid  to  100  cc.  alcohol). 
Since  acids  are  incompatible  with  the  stain,  it  is  best  to  wash 
the  section  next  in  alcohol  or  water  containing  a  trace  of  am- 
monia before  making  the  final  mount. 

Hsematoxylin  is  an  excellent  nuclear  and  cellulose  stain.  It 
scarcely  stains  lignified  material.  Alcoholic  sections  should 
first  be  washed  well  in  water  and  also  thoroughly  washed  after 
staining. 

Beale^s  Carmine. — Dissolve  0.6  gram  of  carmine  in  2  cc.  of 
hot  ammonia  water ;  let  the  solution  stand  1  to  2  hours  to  cool 
and  to  allow  the  excess  of  ammonia  to  escape.  Then  add  60  cc. 
of  distilled  water,  60  grams  of  glycerin  and  15  grams  of  abso- 
lute alcohol.  Allow  the  mixture  to  stand  for  some  time  and 
then  filter  it.  Over-stained  sections  are  brought  back  to  proper 
color  by  washing  in  acidified  70  per  cent,  alcohol,  then  in  alcohol 
free  from  acid.     Carmine  is  a  protoplasmic  and  nuclear  stain. 

FucHSiN. — Dissolve  0.1  gram  of  fuchsin  in  160  cc.  of  water, 
add  1  cc.  of  absolute  alcohol.  Keep  in  a  well-closed  bottle. 
Fuchsin  stains  lignified  and  corky  tissues,  but  is  easily  washed 
out  of  cellulose  walls. 

Methyl-green. — Dissolve  the  dye  in  water  to  deep  green 
color.  This  stains  lignified  and  cutinized  tissues  more  rapidly 
than  cellulose  tissue.  It  also  stains  protoplasm  and  the 
nucleus. 

Safranin. — Equal  parts  by  volume  of  aniline  water  (water 
saturated  with  aniline  oil)  and  concentrated  alcoholic  solution 
of  safranin.  Sections  stained  and  then  washed  in  acidified  70 
per  cent,  alcohol  have  only  lignified  and  cutinized  walls  colored. 

Gentian-violet. — Three  parts  by  weight  of  aniline,  1  part  of 
gentian-violet,  15  parts  by  weight  of  alcohol  and  100  cc.  of 
water.     It  stains  lignified  and  cutinized  walls. 


102  Vegetable  Histology. 

Iodine-green. — A  deep  green  aqueous  solution  is  used.  It 
acts  like  methyl-green.  It  is  much  employed  along  with  car- 
mine, fuchsin  or  eosin  for  double  staining  of  tissues.  The 
stains  are  better  used  successively  than  mixed  together. 

EosiN. — Oil  of  cloves  solution  of  eosin  is  used  for  clearing 
and  at  the  same  time  staining  sections  that  have  previously  been 
treated  with  gentian- violet,  iodine-green  or  methyl-green.  The 
violet  or  green  goes  to  the  lignifled  and  cutinized  tissues,  while 
the  cellulose  walls  are  stained  red  by  the  eosin. 

PiCRO-NiGROSiN  SOLUTION. — Add  euough  of  a  strong  aqueous 
solution  of  nigrosin  to  a  saturated  solution  of  picric  acid  in 
water  to  produce  a  deep  olive-green  color.  This  is  a  good 
nuclear  stain  and  a  good  double  stain,  the  nigrosin  going  to  the 
cellulose  and  the  picric  acid  to  the  lignified  tissues.  A  compara- 
tively long  time  is  required  for  staining. 

Temporary  Stains. 

Potassium  Iodide-iodine. — Dissolve  1  part  of  iodine  and  4 
parts  of  potassium  iodide  in  10  cc.  of  water,  then  dilute  with  185 
parts  of  water.  It  is  one  of  the  most  useful  stains.  It  colors 
starch  blue,  protoplasm  and  proteids  yellowish-brown,  lignified 
cell-walls  deep  brown;  together  with  sulphuric  acid  it  stains 
cellulose  blue. 

Chlor-zinc-iodine. — Dissolve  10  grams  of  potassium  iodide 
and  0.15  gram  of  iodine  in  10  cc.  of  water.  Add  this  solution  to 
100  grams  of  a  solution  of  zinc  chloride  of  specific  gravity  1.8 
and  mix  thoroughly.  This  reagent  may  be  used  either  on  fresh 
or  alcoholic  material,  and  the  specimen  on  the  slide  should  be 
nearly  dried  before  applying  it.  Cellulose  is  colored  (often 
slowlj^)  blue  or  violet,  lignified  walls  yellow,  cork  yellow  to 
brown,  protoplasm  brown,  and  starch  swells  and  is  colored  blue. 

Phloroglucin. — A  solution  of  1  gram  in  100  cc.  of  90  per 
cent,  alcohol.  The  solution  in  time  turns  dark  and  should  not 
be  kept  more  than  three  or  four  months.  The  section  is  first 
immersed  in  the  reagent,  say  for  five  minutes,  after  which  it  is 
nearly  dried  and  a  drop  of  strong  hydrochloric  acid  added  to  it. 
Only  lignified  substance  is  colored.  The  color  varies  from  pale 
to  dark  red,  according  to  the  amount  of  lignification. 

Aniline  Hydrochloride. — A  5  per  cent,  alcoholic  solution  is 
employed  in  the  same  way  as  phloroglucin,  with  hydrochloric 
acid,  as  a  test  for  lignified  tissues,  which  it  stains  a  deep  yellow. 
It  is  not  as  good  a  stain  as  phloroglucin. 

Cyanin. — A  solution  of  cyanin  in  equal  parts  of  alcohol  and 
water  is  used  to  test  for  fats,  which  are  colored  a  beautiful  blue 
after  one-half  hour's  immersion.  Glycerin  may  be  used  to 
wash  out  the  superfluous  stain. 


Reagents.  103 

Alcannin.— Macerate  20  grams  of  alcanet  root  in  100  cc.  of 
90  per  cent,  alcohol  for  a  week  and  filter.  Dilute  the  tincture 
with  an  equal  volume  of  water  just  before  using  and  immerse 
the  sections  for  several  hours.  The  reagent  is  a  test  for  fats, 
resins  and  volatile  oils,  which  assume  a  red  color.  Cutinized 
and  suberized  cells  are  also  stained  red,  though  not  so  deeply. 

Ammonium  Ferric  Alum. — A  saturated  aqueous  solution  is 
used  as  a  test  for  tannins,  w^hich  form  a  bluish-black  or  green- 
ish-black precipitate.  It  should  be  remembered,  however,  that 
occasionally  other  substances,  usually  related  to  the  tannins, 
may  be  present,  which  are  capable  of  forming  dark-colored  pre- 
cipitates with  ferric  salts. 

Fixing  and  Hardening  Reagents. 

Alcohol. — This  is  universally  used  for  hardening  plant  tis- 
sues. It  hardens  by  abstracting  water.  Strong  alcohol  pos- 
sesses also  in  a  high  degree  the  power  of  fixing  the  protoplasmic 
contents  of  cells.  Some  plant  organs  may  be  placed  at  once 
into  absolute  alcohol,  while  other  more  tender  parts  must  he 
placed  at  first  into  weak  alcohol  (60  per  cent.) ,  then  into  grades 
of  increasing  strength,  as  70  per  cent.,  90  per  cent.,  absolute 
alcohol.  Tissues  may  be  kept  in  alcohol  for  any  length  of  time. 
They  acquire  the  best  condition  for  cutting  if  they  are  placed 
in  a  mixture  of  equal  volumes  of  water,  absolute  alcohol  and 
glycerin  24  hours  before  sectioning.  Other  less  frequently  used 
solutions  are — 

Chromacetic  Acid. — A  mixture  of  1  part  of  0.1  per  cent, 
acetic  acid  and  1  part  of  0.2  per  cent,  chromic  acid  solution.  It 
is  a  fixing  reagent  and  objects  must  remain  in  it  from  several  to 
24  hours.  They  are  then  thoroughly  washed  in  water  and  hard- 
ened in  alcohol. 

Chromic  Acid. — A  1  per  cent,  solution  is  used  for  the  same 
purpose  as  the  previous  solution.  All  chromic  acid  mixtures 
should  be  kept  in  the  dark,  as  sunlight  decomposes  them. 

Osmic  Acid. — A  1  per  cent,  solution  fixes  protoplasm  imme- 
diately. Objects  may  remain  in  it  from  a  few  seconds  to  sev- 
eral hours  according  to  their  nature,  and  are  then  washed  thor- 
oughly in  water  and  hardened  in  alcohol. 

Picric  Acid. — In  concentrated  aqueous  or  50  per  cent,  alco- 
holic solution  for  algae  and  higher  plants. 

Softening  Reagents. 

In  some  exceptional  cases  objects  are  too  hard  for  sectioning 
and,  therefore,  must  be  rendered  soft  before  they  can  be  studied. 
Such  objects  are  wood,  hard  seeds,  barks,  dried  drugs.     In  some 


104  Vegetable  Histology. 

cases  mere  soaking  for  a  shorter  or  longer  time  in  cold  or  hot 
water  will  suffice  to  soften  the  specimen.  In  other  cases  weak 
alkalies  are  necessary.  A  very  good  solution  is  2  per  cent,  am- 
monia water  (24  to  48  hours'  immersion).  Very  hard  objects 
are  placed  in  5  per  cent,  caustic  potash  solution. 

Clearing  Reagents. 

To  make  sections  more  transparent  so  that  the  parts  may  be 
better  seen  various  reagents  are  used.  Those  most  generally 
used  are  oil  of  cloves,  creosote^  carbolic  acid  (liquid).  These 
are  used  before  mounting  the  section  permanently  in  balsam  or 
dammar.  Other  reagents  that  are  sometimes  used  are  caustic 
potash  (dilute),  chloral-hydrate,  a  mixture  of  creosote  and  tur- 
pentine (1:3),  or  creosote  and  alcohol  (1:1).  Delicate  objects 
are  gradually  made  clear  even  in  glycerin. 

Starch  is  dissolved  in  dilute  mineral  acids,  protoplasm  in 
dilute  alkalies,  oils  and  resins  in  alcohol,  ether  and  alkalies. 

Alkalies,  acids,  alcohol  or  chloral  hydrate  solution  are  used 
when  it  is  desired  to  clear  out  the  contents  of  cells  so  that  the 
cell-walls  alone  may  be  studied  without  the  interference  of  the 
contents. 

Labarraque^s  Solution  (sodium  hypochlorite)  is  also  used  as 
a  clearing  and  bleaching  agent,  especially  for  cells  rich  in  pro- 
toplasm, for  example,  meristem  cells.  Time  of  action  5  to  15 
minutes.  It  is  excellent  for  bleaching  sections,  etc.,  in  which 
the  natural  plant  pigment  is  too  dark  to  allow  a  clear  view  of 
details.  The  reagent  should  not  be  allowed  to  act  longer  than 
necessary. 

Chloral  Hydrate  Solution. — Chloral  hydrate  crystals,  5 
grams,  dissolved  in  2  cc.  of  water.  It  is  a  very  valuable  reagent. 
It  causes  shrunken  cells  to  expand,  and  dissolves  starch,  pro- 
teids,  resin,  volatile  oils,  chlorophyll,  etc.  When  saturated 
with  iodine,  by  keeping  a  few  crystals  of  the  latter  in  it,  it  is 
employed  to  detect  small  starch  grains. 

Permanent  Mounting  or  Enclosing  Media. 

Canada  Balsam. — This  is  a  thick  solution  of  the  resin  in  ben- 
zene, turpentine,  chloroform  or  xylene.  If  the  solution  becomes 
too  thick  it  is  diluted  with  benzene,  etc.,  respectively.  The 
resin  is  incompletely  soluble  in  absolute  alcohol,  hence  addition 
of  alcohol  to  the  clear  solution  in  benzene,  etc.,  causes  a  cloudi- 
ness. The  balsam  should  be  kept  in  glass-capped,  wide-mouthed 
bottles.  Before  mounting  in  balsam  sections  must  be  soaked  in 
solutions  which  are  miscible  with  it.  Such  solutions  are  clove 
oil,  turpentine,  benzene,  chloroform,  xylene,  creosote,  carbolic 
acid.     It  would  not  do,  for  example,  to  take  a  section  from 


Reagents.  105 

alcohol  into  balsam.  Balsam  hardens  gradually  and  hence  the 
slide  is  finished  when  the  cover  glass  is  placed  on. 

Dammar. — This  is  a  resin  from  which  solutions  are  made 
similar  to  those  of  Canada  balsam,  the  same  kind  of  solvents 
being  used. 

Glycerin-gelatin. — This  is  a  very  convenient  medium  and  is 
often  used  for  mounting  vegetable  sections.  Preparation:  42 
cc.  water,  38  cc.  glycerin,  7  grams  gelatin,  1  gram  carbolic  acid. 
Soften  the  gelatin  (best  French  or  German)  in  the  water  two 
hours,  add  the  glycerin  and  warm ;  add  the  acid,  warm  and  stir 
one-quarter  hour.  Filter  hot  through  glass  wool  and  let  cool. 
It  is  solid  when  cold,  but  melts  at  35°  to  40°  C.,  and  will  keep 
for  years.  Before  mounting  in  this  medium,  tender  objects 
must  be  gradually  brought  from  weaker  to  strong  glycerin. 
The  gelatin  is  then  melted,  a  drop  placed  on  a  warm  slide,  the 
section,  freed  from  most  of  the  adhering  glycerin,  placed  in  it 
and  covered  with  a  warm  cover  glass.  When  cold  the  gelatin 
solidifies.  The  slide  should  then  be  "ringed"  with  a  circle  of 
cement  at  the  edge  of  the  cover  glass.  This  is  done  by  means  of 
a  centering  turn-table,  a  camel's  hair  brush  dipped  in  the  cement 
being  held  at  the  edge  of  the  cover  glass  while  the  slide  rotates 
with  the  turn-table. 

Carmine-stained  sections  cannot  be  mounted  in  gelatin  as  the 
carmine  is  soluble  in  it. 

Farrant^s  Medium. — Equal  parts  by  weight  of  gum  acacia, 
saturated  solution  of  arsenous  acid  and  glycerin.  Soak  acacia 
in  solution  of  arsenous  acid  for  several  days,  then  add  the 
glycerin.  Avoid  shaking,  which  causes  air-bubbles.  The  same 
method  of  mounting  is  employed  in  this  as  in  the  case  of  gly- 
cerin-gelatin.    Slides  should  be  finished  with  a  ring  of  cement. 

Glycerin. — This  is  used  sometimes  as  a  mounting  medium, 
but  is  troublesome  on  account  of  the  difficulty  of  enclosing  it 
with  cement. 

Fluids  for  Temporary  Mounting  of  Objects. 

Water  is  oftenest  used.  Glycerin,  either  concentrated  or 
diluted  to  various  degrees,  is  an  excellent  medium  and  very 
often  used.  A  good  fluid  is  a  mixture  of  equal  parts  by  volume 
of  glycerin,  alcohol  and  water. 

Other  Micro-Reagents. 

Sulphuric  Acid. — Strong  acid  diluted  with  one-fourth  its 
bulk  of  water. 

Phenol  (Carbolic  Acid)  .—Used  as  a  clearing  agent,  also  for 
dehydrating  specimens  when  it  is  not  desired  to  use  alcohol. 


106  Vegetable  Histology. 

Sections  may  be  mounted  directly  from  this  into  balsam. 
Aniline  oil  may  be  used  in  the  same  way.  Aniline  is  kept  free 
from  water  by  placing  in  it  a  stick  of  solid  caustic  potash. 

ScHULZE^s  Maceration  Mixture. — One  gram  of  potassium 
chlorate  dissolved  in  50  cc.  of  nitric  acid,  generally  of  specific 
gravity  1.3,  but  the  strength  of  the  acid  may  be  varied  to  suit 
the  specimen.  It  is  used  for  the  isolation  of  cells.  Sections 
are  placed  in  the  solution  and  gently  heated  until  the  reddish 
color  which  first  appears  in  the  tissue  has  disappeared.  The 
whole  is  then  poured  into  a  large  quantity  of  water  to  stop 
action  and  washed  well  with  water.  The  cells  will  now  be 
found  easy  to  separate.  Sections  should  not  be  carried  from 
alcohol  to  the  mixture,  but  always  from  water,  to  avoid  violent 
action.  Care  is  needed  to  stop  the  action  at  the  right  point. 
The  work  should  be  done  under  a  fume-hood. 

Ammonio-copper  Hydroxide  (Schweitzer's  reagent). — This 
should  be  freshly  prepared  when  needed  by  dissolving  some  pre- 
cipitated copper  carbonate  in  concentrated  ammonia  water. 
The  copper  carbonate  is  obtained  by  adding  sodium  carbonate 
solution  to  a  solution  of  copper  sulphate  and  thoroughly  wash- 
ing and  drying  the  precipitate  by  exposure  to  the  air.  Schweit- 
zer's reagent  is  a  good  solvent  for  cellulose. 

Preserving  Fluids. 

1.  Alcohol.  Pass  objects  from  weaker  to  stronger  solutions 
— 50  per  cent.,  70  per  cent.,  90  per  cent. 

2.  Glycerin.     Pass  from  weaker  to  stronger  glycerin. 

3.  One  per  cent,  solution  carbolic  acid  in  water. 

4.  Aqueous  solution  corrosive  sublimate. 

5.  Aqueous  solution  of  formaldehyde,  2  to  3  per  cent. 

B. — Section  Cutting  and  Procedure  in  Making  a  Permanent 

Mount. 

Section  Cutting. 

Only  very  thin  objects  are  suited  for  examination  under  the 
microscope,  and  the  higher  the  power  the  thinner  must  be  the 
object.  It  is  evident  that  in  order  to  study  large  bodies,  as  the 
organs  of  plants,  thin  slices  or  "sections"  must  be  made.  Such 
sections  should  be  of  as  nearly  equal  thickness  in  all  parts  as 
possible.  A  transverse  section  is  one  at  right  angles  to  the 
long  axis  of  the  object.  A  longitudinal  section  is  one  parallel 
to  the  long  axis  of  the  object.  In  the  case  of  cylindrical  objects, 
as  a  stem,  there  are  two  kinds — 

1.  Longitudinal  radial  section,  which  lies  in  the  plane  of  the 
radius. 


Section  Cutting  and  Permanent  Mounts.  107 

2.  Longitudinal  tangential  section,  which  is  parallel  to  a 
plane  tangent  to  the  cylinder,  and  cuts  the  latter  near  the  sur- 
face. 

The  razor  must  always  be  keen-edged  and  should  be  stropped 
frequently  to  keep  it  thus,  and,  when  necessary,  honed.  It  is 
impossible  to  cut  a  thin  section  with  a  dull  razor.  A  razor  flat 
on  one  side  is  the  best.  It  should  always  be  cleaned  after  cut- 
ting sections.  It  should  be  pushed,  rather  than  drawn,  through 
the  object,  with  an  oblique  or  sliding  motion,  even  and  steady 
and  never  with  a  to-and-fro  or  sawing  motion. 

Sections  may  be  cut  free-hand  or  by  the  use  of  a  so-called 
microtome  or  section-cutter,  a  machine  in  which  the  object  is 
clamped  in  a  jaw,  which  is  raised  by  an  accurate  screw,  while 
the  razor,  either  held  by  the  hand  or  clamped  in  a  carriage, 
slides  through  it,  giving  very  even  and  thin  sections. 

Free-hand  Sections. — If  the  object  is  large  it  is  held  in  the 
left  hand  between  the  thumb  and  forefinger,  the  latter  being 
extended  slightly,  so  as  to  form  a  rest  for  the  razor-blade,  which 
is  held  in  the  right  hand.  Small  objects  are  held  in  elder  or 
sunflower  pith,  which  is  split  longitudinally  in  halves.  The  ob- 
ject is  left  protruding  slightly,  or  both  object  and  pith  are  cut 
together. 

In  most  cases  it  is  best,  in  cutting,  to  keep  the  knife-blade  wet 
with  alcohol  or  a  mixture  of  equal  parts  of  alcohol  and  glycerin. 
Sections  of  fresh  tissues  or  of  those  that  have  been  kept  in  any 
of  the  preserving  fluids  should,  immediately  after  cutting,  be 
transferred — best  by  means  of  a  camePs  hair  brush — to  water 
or  alcohol,  otherwise  air  will  get  into  the  cells  and  seriously 
impair  the  value  of  the  section  for  study.  In  regard  to  cutting 
sections  with  the  microtome,  practice  under  the  eye  of  an  in- 
structor is  the  best  teacher. 

While  in  the  majority  of  instances,  certainly  in  those  men- 
tioned in  this  book,  the  method  of  cutting  sections  above 
described  gives  satisfactory  results,  in  the  case  of  very  delicate 
objects,  such  as  anthers,  very  young  ovaries,  longitudinal  sec- 
tions of  root  tips,  etc.,  more  careful  manipulation  is  required. 
The  objects  are  imbedded  either  in  paraffin  or  celloidin.*  The 
paraffin  method  requires  less  time  and  is  preferred  by  many  to 
the  celloidin  method,  especially  in  animal  histology.  Celloidin 
is  especially  applicable  to  delicate  plant  tissues.  The  two 
methods  are  described  in  Sedgwick  and  Wilson's  General 
Biology,  as  follows : 

Paraffin  Method. — After  hardening  and  staining,  the  object 
is  soaked  in  alcohol  (95  per  cent,  or  more)  until  the  water  is 
thoroughly  extracted  (2  to  12  hours,  changing  the  alcohol  at 
least  once),  then  in  chloroform  until  the  alcohol  is  extracted 

•Celloidin  is  the  best  quality  of  gun-cotton,  and  occurs  in  tlie  market  in  pieces 
looking  somewhat  like  cartilage. 


108  Vegetable  Histology. 

(2  to  12  hours),  and  then  in  melted  paraffin  (not  warmer  than 
55°  C.)  on  a  water-bath  for  15  to  30  minutes  (too  high  a  tem- 
perature or  two  long  a  bath  causes  excessive  shrinkage) .  Some 
of  the  paraffin  is  then  poured' into  a  small  paper  box  or  into  ad- 
justable metal  frames.  The  object  is  transferred  to  it,  and 
after  the  mass  has  begun  to  set  it  is  placed  in  cold  water  until 
quite  hard.  It  is  then  cemented  (by  paraffin)  to  a  square  piece 
of  cork  and  placed  in  the  microtome.  In  cutting  the  knife  is 
kept  dry.  The  sections  should  be  fixed  on  the  slide  by  the  col- 
lodion method.  (Collodion  mixture  consists  of  1  part  of  ether- 
collodion  and  3  parts  of  oil  of  cloves.  In  mounting  sections  a 
slide  is  smeared  with  the  mixture  by  means  of  a  camel's  hair 
brush,  the  sections  laid  on  and  the  slide  placed  on  a  water-bath 
for  a  few  minutes  to  evaporate  the  oil  of  cloves.  The  slide  is 
then  placed  in  turpentine  (to  dissolve  the  paraffin),  then 
drained,  after  which  a  drop  of  balsam  is  placed  on  the  section 
and  a  cover  glass  put  on.) 

Celloidin  Method. — After  dehydrating  the  object  thoroughly 
in  alcohol,  soak  it  24  hours  in  a  mixture  of  equal  parts  of  alcohol 
and  ether.  Make  a  thick  solution  of  celloidin  in  the  same  mix- 
ture and  soak  the  object  for  some  hours  in  it.  It  may  then  be 
imbedded  as  follows :  Dip  the  smaller  end  of  a  tapering  cork 
in  the  celloidin  solution,  allow  it  to  dry  for  a  moment  (blowing 
on  it  if  necessary),  and  then  build  upon  it  a  mass  of  celloidin, 
allowing  it  to  dry  a  moment  after  each  addition.  Transfer  the 
object  to  the  cork  and  cover  it  thoroughly  with  the  celloidin. 
Then  float  the  cork  in  82  to  85  per  cent.  (0.842  specific  gravity) 
alcohol  until  the  mass  has  a  firm  consistency  (24  hours).  It 
may  then  be  cut  in  the  microtome,  the  knife  being  kept  very  wet 
with  alcohol  of  above  strength.  Keep  the  sections  in  85  per  cent, 
alcohol  until  ready  to  mount  them,  then  soak  them  for  a  minute 
in  strong  alcohol,  transfer  to  a  slide,  pour  on  chloroform  until 
the  alcohol  is  removed,  drain  off  the  liquid,  quickly  add  a  drop  of 
balsam  and  cover. 

Operations  Involved  in  Making  a  Permanent  Mount  op  a 

Section. 

Fixing  and  Hardening. — In  many  cases  plant  tissues  are  sec- 
tioned and  examined  in  the  fresh  state.  But  often  the  contents 
of  cells  in  the  living  state  are  too  transparent  to  be  seen  dis- 
tinctly and  must  be  killed  in  order  to  make  them  more  opaque 
and  easily  seen  under  the  microscope.  Again,  portions  of 
plants  may  be  too  soft  to  be  cut  without  first  being  put  through 
a  hardening  process.  In  all  cases  where  it  is  desired  to  make 
permanent  mounts  or  slides  of  tissues  the  protoplasm  is  first 
killed  and  then  hardened,  unless  the  tissue  consists  of  cells 
already  dead,  for  example,  wood  cells,  stone  cells  of  nuts,  etc. 


Section  Cutting  and  Permanent  Mounts.  109 

The  object  of  killing  the  protoplasm  is,  as  just  stated,  to  make 
it  more  opaque,  and,  at  the  same  time,  to  preserve  its  structure 
for  a  long  time.  The  process  is  termed  fixing.  The  protoplasm 
is  fixed  or  coagulated. 

After  fixing  the  object  must  often  be  hardened  for  cutting. 
There  are  various  reagents  for  fixing  and  hardening,  some  of 
which  do  both  at  the  same  time,  while  others  only  fix  or  kill. 
(See  under  the  respective  reagents  for  fixing  and  hardening 
tissues,  page  103.) 

Although  there  are  a  number  of  such  reagents,  those  actually 
used,  especially  in  botany,  are  few.  Alcohol  is  used  in  nearly 
all  cases.  Tender  objects  must  never  he  placed  at  once  in  strong 
alcohol. 

Staining. — It  often  happens  that  some  objects,  even  after 
fixing,  are  so  transparent  that  their  structure  cannot  easily  be 
made  out.  The  more  a  transparent  body  approaches  in  its 
refraction  of  light  the  medium  in  which  it  lies  the  more  difficult 
it  is  to  be  seen.  Finally,  when  the  refracting  power  is  the  same 
as  that  of  the  medium,  the  object  is  invisible.  Shells  of 
Diatoms  in  glycerin  are  invisible,  the  refractive  indices  of  both 
being  1.43.  By  staining  or  coloring  the  transparent  parts  of  an 
object  these  become  visible  and  their  structure  is  easily  made 
out.  In  a  heterogeneous  section,  like  that  of  a  plant  stem,  for 
example,  the  chemically  different  materials  in  it  select  differ- 
ent stains,  so  that  by  a  contrast  of  colors  a  great  deal  more  is 
learned  than  by  study  of  the  unstained  section.  The  different 
stains  require  different  lengths  of  time  for  action,  which  is  best 
learned  by  actual  work  with  them.  Some  of  them  are  perma- 
nent, others  only  temporary.  The  stains  fall  into  groups,  as 
aniline,  carmine,  haematoxylin  stains,  etc. 

Clearing. — In  most  cases,  even  after  making  very  thin  sec- 
tions, these  are  too  opaque  and  obscure  for  observation  with  the 
higher  powers  of  the  microscope  and,  consequently,  must  be 
made  more  transparent,  that  is,  must  be  cleared.  Some  objects 
are  naturally  very  opaque,  as  pollen  grains,  spores ;  such  objects 
must  always  be  cleared  before  studying  them.  It  often  hap- 
pens that  it  is  desired  to  study  the  cell-walls  only  of  a  section, 
but  this  is  impossible  because  of  the  dense  mass  of  cell  contents, 
as  starch,  protoplasm,  oil,  resin,  milk-sap,  etc.,  which  must  first 
be  removed  by  special  clearing  reagents.  (See  the  various 
clearing  reagents,  page  104.) 

Mounting. — If  objects  are  not  intended  to  be  kept  for  a  long 
time  they  are  mounted  in  water,  alcohol,  dilute  glycerin  or  con- 
centrated glycerin,  or  some  other  suitable  material.  For  per- 
manent mounting  they  are  usually  placed  in  a  medium  which 
solidifies  after  a  time,  such  as  balsam,  glycerin-gelatin,  which 
are  commonly  used.  The  handiest  is  the  second,  because  of  the 
little  preliminary  treatment  necessary.    The  object,  in  what- 


110  Vegetable  Histology. 

ever  medium  it  is  to  be  mounted,  must  be  saturated  with  a  fluid 
not  very  different  from  the  mounting  medium.  For  example, 
to  mount  in  gelatin,  the  object  must  be  transferred  from  gly- 
cerin; in  balsam,  from  cloves,  turpentine,  etc.;  in  Farrant's 
medium,  from  glycerin.  Media  which  do  not  harden  or  which 
absorb  moisture,  as  glycerin-gelatin,  must  be  closed  in  with  a 
ring  of  cement.     (See  mounting  media,  page  104.) 

Scheme  for  Making  Permanent  Mounts. 

The  specimens  are  supposed  to  be  alcoholic ;  if  not,  they  are 
to  be  placed  for  a  time  in  dilute  alcohol.  Alcoholic  objects  take 
the  stains  better  than  fresh  tissues.  In  the  following  scheme 
the  sections  are  supposed  to  be  free  from  any  imbedding  mate- 
rial. The  procedure  for  mounting  paraffin  or  celloidin  sections 
has  already  been  stated  under  Section  Cutting. 

Balsam  Mounts. — If  the  section  be  lying  in  alcohol — 

1.  Wash  in  water  if  it  is  to  be  stained  in  aqueous  stains. 

2.  Transfer  to  staining  liquid.  Choose  stains  according  to 
the  nature  of  the  cells  that  it  is  desired  to  stain.  Aniline  stains 
color  chiefly  woody  or  lignified  material.  If  the  stains  are  in 
alcoholic  solutions  take  objects  from  alcohol  of  about  same 
strength.  The  time  required  will  vary  according  to  the  nature 
and  strength  of  the  stains  and  is  best  learned  by  actual  practice 
with  them.  The  sections  should  be  examined  at  intervals  for 
depth  of  color  by  transferring  them  to  water  or  alcohol  and  then 
back  again  to  the  staining  liquid.  After  one  has  stained  a  few 
sections  he  obtains  an  idea  of  the  exact  time  required  to  attain 
proper  color  effects. 

3.  Wash  off  the  superfluous  stain  by  moving  the  section 
about  in  water. 

4.  Dehydrate  the  section  by  passing  it  into  70  per  cent, 
alcohol  3  minutes,  90  per  cent,  alcohol  5  minutes,  absolute  alco- 
hol (distilled  from  lime)  5  or  more  minutes. 

5.  Place  the  section  next  in  a  clearing  agent,  as  oil  of  cloves 
or  turpentine,  5  minutes  or  more.  If  section  be  not  completely 
dehydrated  it  will  now  appear  opaque.  It  must  then  be  passed 
back  to  the  absolute  alcohol  and  back  again  to  the  clearing 
reagent. 

6.  Transfer  the  section  from  the  clearing  agent  to  a  slide 
and  remove  the  excess  of  the  former.  Place  upon  it  a  drop  of 
balsam  and  then  a  cover  glass,  avoiding  air-bubbles. 

Glycerin-Gelatin  Mounts. — Steps  1,  2,  3  are  the  same  as 
above. 

4.  Pass  the  section  into  weak  glycerin  3  to  5  minutes. 

5.  Then  into  stronger  glycerin  3  to  5  minutes. 

6.  From  5  into  concentrated  glycerin  5  minutes. 


Test  Reactions.  Ill 

7.  Remove  excess  of  glycerin  and  mount  in  glycerin-gelatin, 
as  directed  under  the  latter  medium. 

8.  Enclose  the  cover  glass  with  a  ring  of  cement  (asphaltum, 
Brunswick  black,  etc.). 

C. — More  Important  Test  Reactions  of  the  Parts  op  Vege- 
table Cells.     (Bower^s  Practical  Botany.) 

Cellulose  cell-walls — 

1.  Colored  faintly  yellow  by  iodine. 

2.  Swollen  and  ultimately  dissolved  by  sulphuric  acid. 

3.  Colored  blue  with  iodine  and  sulphuric  acid. 

4.  Colored  blue  or  violet  with  chlor-zinc-iodine. 

5.  Stained   by   solutions   of   carmine   or  hsematoxylin,   by 

methylene-blue  and  in  various  degrees  by  other  aniline 
colors. 

LiQNIFIED    cell-walls 

1.  Colored  distinctly  yellow  by  iodine  and  by  chlor-zinc- 

iodine,  but  in  case  of  bast  fibres  the  tint  may  vary  to 
sherry-brown  or  even  pink. 

2.  Colored  brown  and  swollen  by  iodine  and  sulphuric  acid. 

3.  Colored  bright  yellow  by  acidulated  solution  of  aniline 

sulphate. 

4.  Colored  red  with  acid  solution  of  phloroglucin. 

5.  Stained  slightly  or  not  at  all  by  solutions  of  carmine  and 

hgematoxylin,  but  readily  by  aniline  colors. 

CUTICULARIZED    OR   CORKY    CELL-WALLS 

1.  Yellow  by  iodine. 

2.  Yellow  or  brown  by  chlor-zinc-iodine. 

3.  Yellow  by  strong  potash;  on  gradually  warming  (without 

boiling)  bright  yellow. 

4.  Resist  action  of  sulphuric  acid,  retaining  their  clearly- 

marked  outlines. 

5.  Are  not  stained  by  solutions  of  carmine  or  hsematoxylin, 

but  are  colored  by  aniline  stains. 
Mucilaginous  walls — Resemble  cellulose  in  many  reactions. 

1.  Swell  with  water  and  to  a  greater  extent  with  potash. 

2.  Do  not  stain  with  iodine. 

3.  Stain  red  with  Hanstein's  aniline-violet,  blue  with  methyl- 

ene-blue. 
Calcium  oxalate — Occurs  in  cells  in  form  of  crystals. 

1.  Insoluble  in  acetic  acid. 

2.  Soluble  without  evolution  of  gas  in  nitric  or  hydrochloric 

acids. 

3.  Soluble  in  sulphuric  acid,  with  formation  of  fresh  crys- 

tals of  calcium  sulphate  if  only  small  bulk  of  fluid  be 
present. 

4.  Are  not  stained  with  iodine. 


112  Vegetable  Histology. 

Calcium  carbonate — Occurs  as  incrustations  or  crystals;  it  is 
soluble  in  acetic  acid  with  evolution  of  gas  (COg). 

Protoplasm  or  proteids  generally — 

1.  Yellow  or  brown  by  iodine  solutions. 

2.  Yellow  by  nitric  acid ;  on  addition  of  potash  or  ammonia 

a  bright  yellow  color  is  produced  (xanthoproteic  reac- 
tion). 

3.  Swell  and  lose  details  of  structure  on  treatment  with 

potash,  ammonia  or  Labarraque's  solution. 

4.  Stain  readily  with  carmine,  hsematoxylin,  bright  red  with 

Hanstein's  aniline-violet. 

5.  Best  stains  for  nucleus  are  haematoxylin,  safranin  and 

methyl-green. 

Starch  grains — 

1.  Blue  with  solutions  of  iodine  in  presence  of  water. 

2.  Swell  in  potash  and  in  water  above  65°  C. 

3.  Swell  in  dilute  sulphuric  acid. 

4.  Swell  and  are  colored  blue  with  iodine  in  chloral  hydrate. 

Inulin — 

1.  Soluble,  but  not  readily,  in  cold  water. 

2.  Precipitated  as  sphere-crystals  by  alcohol  or  glycerin. 

3.  Not  colored  by  iodine,  and  soluble  in  potash. 

Fixed  oils — 

1.  Black  with  osmic  acid. 

2.  Saponified  with  potash ;  soluble  in  ether. 

3.  Pink  with  alcannin  solution. 

Resin — 

1.  Soluble  in  alcohol  or  ether. 

2.  Red  by  alcannin  solution  and  blue  by  Hanstein's  aniline 

violet. 

Tannin — 

1.  Deep  brown  by  potassium  dichromate  or  chromic  acid. 

2.  Greenish  blue  by  ferric  salts. 


582810 


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